The atmosphere is a huge cloud of gas surrounding Earth; it becomes thinner as you get closer to outer space. The atmosphere is held into position by gravity, while the Sun also helps keep the atmosphere in place. If there was no Sun, the atmosphere would freeze and collapse to Earth, which would then lead to harmful UV radiation as the Earth moved in a straight line through outer space. Global temperatures would plunge to absolute zero within a couple of weeks, killing all life on Earth, except for some types of bacteria, such as archaea, which live in extreme environments.
THE COMPOSITION OF THE ATMOSPHERE
The dry parts of the atmosphere include nitrogen, oxygen, argon, carbon dioxide, and other gases. Nitrogen, oxygen, argon, and CO2 are the most abundant gases in the atmosphere, in total making up over 99% of it, with nitrogen being the most common, at 78%.
Nitrogen dilutes oxygen and it prevents the Earth’s surface from burning up.
Oxygen is necessary for respiration and it is also necessary for things to burn or combust.
Argon preserves paper and it is also used in light bulbs.
CO2 is a greenhouse gas, just as NOAA says it is. CO2 does in fact keep some heat from escaping into outer space, in which the heat is absorbed by the CO2 molecules, and then gets reflected back to Earth’s surface and heats it again. However, to the contrary of NOAA‘s and NASA‘s beliefs, CO2 is a very weak greenhouse gas.
According to a chemistry teacher I know, “the CO2 molecule is very rigid due to its double bond structure and very low polarity due to symmetry. CO2 is a lousy greenhouse gas. Nitrogen and Oxygen are NOT greenhouse gases due to their double and triple bond structures, and even lower polarity. CO2, unlike O2 and N2, does have some greenhouse characteristics, [which makes it a greenhouse gas], but as a greenhouse gas, it is very weak. It just happens to be the second most abundant greenhouse gas in the atmosphere. The only reason CO2 is on the radar is due to its abundance in the atmosphere. If CO2 was a major contributor [to global warming], we would see the results quite vividly. The fact that we don’t proves it is not.”
In addition, carbon dioxide only makes up 0.04% (400 ppm) of Earth’s atmosphere. Prior to the industrial revolution, CO2 levels were relatively steady for about 11,000 years, with natural variation. When CO2 levels rise or fall by 100 ppm, the process generally takes between 5,000 and 20,000 years. However, since the industrial revolution, human emissions have increased the CO2 level by 100 ppm in just 200 years. However, do not fret! Over the past 500,000 years, Earth’s temperature has been the control knob for CO2 levels. The common climate alarmism myth today is that CO2 controls the temperature. As you can see by the graph below, temperature rises first, then CO2 will eventually follow. Please also note that while CO2 levels have skyrocketed since the late 1700s, the temperatures have not responded – yes temperatures have risen, most likely due to natural factors and urbanization, but they are not outside of the natural boundary of where they should be at this point in time. In fact, the previous four interglacial periods have all been warmer than the current one according to proxies.
If we look back even further in the geological record, going back 600 million years, we will see that there is no linkage between CO2 and temperature. We have had ice ages with CO2 levels at 5,000 parts per million; we have had warmer times with less CO2 and colder times with more CO2. The recent 500,000 year trends of temperature controlling the levels of CO2 has no explanation.
So, how much CO2 do humans emit per year? If we look at the Global Carbon Cycle, humans emit about 29 gigatons of CO2 each year. Compare that to the approximate 750 to 800 gigatons of CO2 in total emitted each year from both human and natural processes. Vegetation emits 439 gigatons of CO2 each year, while the oceans emit 332 gigatons of CO2 per year. Because humans emit the extra 29 gigatons of CO2, the oceans and land surface (vegetation) can only absorb about 41% of the human-emitted CO2. The other 59% remains in the atmosphere.
Because human emissions only account for 3.5 to 4% of all of the CO2 in the atmosphere, we can only conclude that any global warming taking place has to be influenced by the natural climate system, which includes the Sun and the oceans. Because CO2 is a weak greenhouse gas as explained above, and because it is such a small part of the atmosphere (even though it is the most abundant greenhouse gas), the solar energy that CO2 absorbs is very little and the heat reflected back to Earth’s surface is even less than the amount absorbed. We can also look at the ratio of carbon isotopes in the atmosphere to see that recent rising CO2 levels have been due to human activity. Carbon isotopes are carbon atoms with each having a different number of neutrons. For instance, C12 has six neutrons, while C13 has seven. Vegetation have a lower C13/C12 ratio than the atmosphere does, which means that C13/C12 ratios in the atmosphere should fall, which is what is currently occurring. This trend correlates with global fossil fuel emissions.
The only greenhouse gas that should be on the radar is water vapor, because it is much more abundant than CO2, and it is a much stronger greenhouse gas than CO2. If there were to be a substantial increase in water vapor, then that would be somewhat alarming, but luckily for us, that increase has been small and because the climate system is designed so well, increases in water vapor will not do much to the unbalance the climate system.
Water vapor can make up to 4% of the atmosphere at maximum and 0% at minimum, depending on the region of the Earth you are looking at. In deserts, there are dry winds, which keep the H2O concentration at 0%. In tropical climates, the levels are at 3%, pushing closer to 4% closer the equator. In the past 40 years or so, there has been a slight increase in water vapor near the north pole, which has contributed to the slight Arctic warming we have been seeing since 1979 during the winter months.
THE LAYERS OF THE ATMOSPHERE
The atmosphere has five different layers, each separated by a ‘pause.’
- The outermost layer of Earth’s atmosphere.
- Extends from the top of the thermopause to 6,000 miles (10,000 km) above the Earth.
- Atoms and molecules escape to outer space in this layer.
- Satellites orbit the Earth here.
- Separated from the thermosphere by the thermopause, which is around 375 miles (600 km) above Earth.
- Extends from the top of the mesopause to 375 miles (600 km) above Earth.
- Extremely thin, but becomes denser as you get closer to Earth’s surface.
- UV radiation from the Sun gets absorbed by the air molecules in the thermosphere, which causes a huge temperature increase from -184°F (-120°C) at the bottom, to 3,600°F (2,000°C) at the top.
- The extremely hot temperature would actually feel really cold to our body, because the thermosphere has extremely thin air. There are less molecules in the air, so the amount of energy absorbed by the molecules is not enough to heat our skin.
- Within the thermosphere, lies the ionosphere, which extends between 37 miles (60 km) and 190 miles (300 km) above Earth’s surface.
- It is divided into three layers; the F-Layer, E-Layer, and D-Layer.
- During the daytime, the F-Layer splits into two layers, then it recombines during nighttime.
- The E-Layer was discovered in 1901, while Marconi was transmitting a signal between Europe and North America. The signal showed that it had to bounce off of an electrically conductive region, approximately 62 miles (100 km) above Earth’s surface. The region was named the E-Layer (Electrical-Layer) in 1927 by Sir Edward Appleton. In addition to the E-Layer, the F-Layer and D-Layer were discovered later on.
- The ionosphere exists because of the UV radiation being absorbed by molecules.
- The density of the ionosphere changes from day to night. All layers of the ionosphere are denser during the day because of the Sun. The opposite happens at night, and the layers become less dense, with the D-Layer simply disappearing altogether.
- The ionosphere is important, because it bounces radio signals transmitted from a radio.
- Extends from the stratopause, about 31 miles (50 km) to 53 miles (85 km) above the Earth’s surface.
- Temperatures increase as you get closer to Earth, because the air gets denser; with the temp rising to about 5°F (-15°C).
- The molecules here are now dense enough to slow down meteors and meteorites.
- Extends from the tropopause, about 4 to 12 miles (6 to 20 km) above the surface to 31 miles (50 km).
- The stratosphere consists of most of the gases in the atmosphere, except for water vapor. (There is only a little bit of water vapor in the stratosphere).
- Unlike the mesosphere, the temperature increases with height here. The temperature goes from -60°F (-51°C) at the bottom to 5°F (-15°C) at the top.
- The increase in temperature means that warm air is atop the cooler air. This prevents convection because there is no vertical movement of the gases in the atmosphere. You can easily see the bottom of the stratosphere by looking at the anvil-shaped top of a cumulonimbus cloud.
- Extends from Earth’s surface to about 4 to 12 miles (6 to 20 km) above the Earth’s surface depending on the terrain and shape of the atmosphere surrounding the Earth. At either pole, the top of the troposphere is about 4 miles (6 km) above the surface. At 50°N and 50°S, it is about 5.5 miles (9 km) above the Earth’s surface, and at the equator, it is about 12 miles (20 km).
- This layer is known as the lower atmosphere.
- All weather occurs here.
- Temperature decreases with height; it goes from about 59°F (15°C) to -60°F (-51°C).
The atoms and molecules in the air that make up the atmosphere constantly move. When they touch a surface, they apply a force on the surface of that object. We call this pressure. Even though each atom and molecule are tiny, a large number of them can exert a noticeable force when striking a surface.
Air pressure can be increased or decreased in two ways. The first way to increase air pressure is by adding molecules to a container. A greater quantity of molecules will increase the number of collisions with the container’s boundaries, which will increase the pressure. This can be observed by pumping air into a tire. When pumping air into a tire, the number of molecules will increase, which will in turn increase the number of collisions with the tire’s boundaries. The increase in collisions makes the tire pressure higher, which causes the tire to inflate, or expand. To decrease the air pressure, you simply do the opposite of what you do to increase the pressure.
The other way to increase or decrease air pressure is by adding or subtracting heat. The addition of heat to an object will cause the object to expand. This is because the heat adds energy to air molecules. The increased energy in air, increases the number of collisions with an object’s boundary, which increases the pressure.
As elevation increases, the number of air molecules decrease, which makes the air less dense. This causes a decrease in atmospheric pressure. One half of the total air molecules in the Earth’s atmosphere are within 18,000 feet (5.6 km) of Earth’s surface.
The decrease in air pressure with height makes it difficult to compare the air pressure at ground level from location to location because of elevation. In order to compare the air pressures at any two or more given locations, there has to be a common denominator to which we convert the air pressure at that location to the common denominator. This is done by converting it to sea-level elevation. The number we would get from doing the conversion would be equal to the air pressure reading if the station were to be at sea-level.
The most common units to measure air pressure in the United States are by inches of mercury and millibars.
According to the National Weather Service,
“Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. At sea level, standard air pressure is 29.92 inches of mercury.”
“Millibars comes from the original term for pressure “bar.” Bar is from the Greek “báros” meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.”
Air pressure is constantly changing. The changes in air pressure are due to changes in air density. Air density is closely related to temperature. Cooler air is denser than warmer air because the molecules in warmer air have a greater velocity. They are also farther apart in warmer air than in cooler air. While the average altitude of the 500 millibar level is approximately 18,000 feet (5,600 m), the elevation will be higher in warm air than in cooler air.
Air pressure also changes due to moving weather systems. These systems are easily identified on a weather map. The blue H represents and area of high pressure and the red L represents an area of low pressure.
So how do changes in weather influence air pressure changes? In 1948, Reverend Dr. Brewer of England wrote this in A Guide to the Scientific Knowledge of Things Familiar.
“The FALL of the barometer (decreasing pressure)
- In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind.
- In frosty air, the fall of the barometer denotes thaw.
- If wet weather happens soon after the fall of the barometer, expect but little of it.
- In wet weather if the barometer falls expect much wet.
- In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and possibly wind.
- The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind).
The RISE of the barometer (increasing pressure)
- In winter, the rise of the barometer presages frost.
- In frosty weather, the rise of the barometer presages snow.
- If fair weather happens soon after the ruse of the barometer, expect but little of it.
- In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.
- In wet weather, if the mercury rises suddenly very high, fine weather will not last long.
- The barometer rises highest of all for north and east winds; for all other winds it sinks.
The barometer UNSETTLES (unsteady pressure)
- If the motion of the mercury be unsettled, expect unsettled weather.
- If it stands at “MUCH RAIN” and rises to “CHANGEABLE” expect fair weather of short continuance.
- If it stands at “FAIR” and falls to “CHANGEABLE”, expect foul weather.
- Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather.”
Rev. Dr. Brewer’s observations are true for many locations around the globe, but not all.
While the weather any one particular place or area receives changes day to day, sometimes considerably, there is an overall trend that we see year after year. This recurring weather over a long period of time is called climate.
A German climatologist named Wladimir Köppen (1846-1940) worked with Rudolf Geiger to create climate zones based off the average temperature per region and other factors. There are six major categories; A, B, C, D, and H.
A: TROPICAL CLIMATES
Tropical climates extend both north and south of the equator to 15° to 25° in latitude. Temperatures in these regions average over 64°F (18°C) year round and annual precipitation is over 59 inches (1.5 m).
B: DRY CLIMATES
Dry climates are characterized by being dry…obviously; they extend from 20° to 35° North and South of the equator. They are also found on large land masses or continents in mid-latitude, surrounded by mountain ranges. In these climates, evaporation outdoes precipitation.
C: MOIST SUBTROPICAL MID-LATITUDE CLIMATES
These climates tend to be warm, with humid summers and mild winters. They extend from 30° to 50° in latitude, typically on the eastern and western borders of continents.
D: MOIST CONTINENTAL MID-LATITUDE CLIMATES
These climates have warm to cool summers, with very cold winters. These climates are above (Northern Hemisphere) and below (Southern Hemisphere) moist subtropical mid-latitude climates. Average temperatures during the summer are around 50°F (10°C).
E: POLAR CLIMATES
Polar climates are bitterly cold nearly year-round with the warmest month of the year averaging less than 50°F (10°C). These climates are found on the northern coastal areas of North America, Europe, Asia, and in Greenland, the Arctic, and Antarctica.
Highlands are based on their high elevation; they are in very high mountain ranges where rapid elevation increases, which causes a quick change in climate over short distances.
Below is a map from the National Weather Service, which shows the main climate zones of the United States.
Below is another map of climate zones, but for the globe, and much more detailed into sub-categories.
THE SUN’S MOVEMENT OF ENERGY & HEAT
The main driver of the climate system is the Sun. The Sun gives out energy to the planets in the solar system. This energy comes from TSI (Total Solar Irradiance) and sunspot count. The energy is given out by the Sun is energy in motion, otherwise known as kinetic energy. Temperature is a measurement of the amount of kinetic energy possessed by the particles of an object. The more energy that is given out by the Sun means that Earth will be warmer. The less energy given out by the Sun means that Earth will cool. There are three ways for which this energy is transferred through Earth’s atmosphere. They are radiation, conduction, and convection.
When the Sun’s rays enter the atmosphere, this is known as radiation. Some of this energy given out by the Sun is absorbed by the atmosphere, some gets reflected back into outer space by clouds, while some of it reaches the Earth’s surface. The rest of the energy gets absorbed by plants for the process of photosynthesis. The energy that is absorbed by the atmosphere will heat up the atmosphere. The energy that reaches Earth’s surface is an example of radiation. The energy heats the surface, then gets reflected back into outer space. The heat reflected helps heat the air, which is an example of conduction.
Most of the energy that is reflected off of the Earth’s surface is sent back into outer space. Some of the energy, however, gets trapped in the atmosphere as extra heat energy. Greenhouse gases like water vapor, ozone, methane, and carbon dioxide absorb this heat. Those greenhouse gases reflect the energy back to Earth’s surface and heat it again, while a portion of it is once again used for photosynthesis. This extra energy getting reflected back to Earth may result in more plant growth. As we have previously stated above, CO2 has very little effect on the global climate system.
Sooner or later, all of the energy given out by the Sun to Earth will escape back into outer space.
The solar energy the Earth receives is not evenly distributed around the globe. This is because of the Earth’s shape and the tilt of the Earth’s axis. The amount of energy received by Earth changes at different latitudes. This changes from season to season. The poles receive the least amount of solar energy while the Equator gets the majority of it, which is why the equator is hot and the poles are cold.
The climate system likes to try and balance the unevenly distributed energy by moving it around the globe, particularly from the equator to either pole. The oceans and air currents help move this energy around daily.
Near the equator, there is more energy in the atmosphere. This allows thunderstorms to develop. The cumulonimbus clouds force warm air to rise. The warm air will move toward the north or south pole depending upon atmospheric conditions.
The rising warm air cools with altitude. As the air cools, low pressure forms. This lower atmospheric pressure makes temperatures cooler and the weather stormier. When the air circulates downward, the air compresses, which increases the number of air molecule collisions with the “boundary” which increases the air pressure. High pressure usually makes the weather dry and hot, but fair. In between each of the circulations lies areas of high and low pressure. High pressure areas are located at 30° N and at 30° S latitude and at each pole. These areas tend to house deserts (Fun Fact! Antarctica is a desert too). Low pressure bands are located at 50° to 60° N and 50° to 60° S latitude and also at the equator. These areas, especially on the west coasts of continents tend to have lots of precipitation because of the way storms move around those areas. This pattern of circulating air forms cells.
There are three circulations flowing toward each pole due to the Earth’s rotation.
- Hadley Cell | Lower latitude air moving toward the equator. With heating, it rises vertically, then moves poleward in the upper atmosphere.
- Ferrel Cell | Mid-latitude air circulation where air flows toward the poles and eastward close to the surface, and flows toward the equator and westward in upper air.
- Polar Cell | Air rises, then diverges and travels poleward. The air cools as it moves northward or southward. As it cools, the air sinks over the poles, compressing the air, which is what forms the polar highs. Once the air reaches the surface, the air diverges outward from the polar highs. Surface winds also known as polar easterlies, flow eastward.
This circulating air in the atmosphere is what causes wind. The air in the atmosphere acts like a fluid in the process of convection. The Sun’s radiation reaches the Earth’s surface, which warms the ground. As the Earth’s surface warms due to conduction, the heat from the ground will get reflected back into the air. This forms a bubble of warm air, which is surrounded by cooler air. As the air rises, the ‘bubble’ itself cools with the heat still inside. Eventually, the warm air will get replaced by the cool air surrounding it. The cooler air is denser, which sinks to the bottom, as we just stated. The warm air getting replaced with cool air is felt by us as wind. This circulating pattern of warm air forms cells all over the globe. These cells keep the heat moving around the atmosphere. The movements of these air masses can be localized, such as in cumulus clouds, or they can cover a large area, in the cells. These circular movements are a big part of the weather patterns in Earth’s atmosphere.
WINTER & SUMMER SOLSTICE, AUTUMN & SPRING EQUINOX
As we have previously discussed, the equatorial region of Earth receives the majority of the Sun’s energy, but not always. The Earth’s axis is titled at 23½°. The amount of solar radiation that a certain place receives varies from season to season. In the Northern Hemisphere winter, the Southern Hemisphere is in its summer. Therefore, the Northern Hemisphere receives the lesser amount of solar energy, while the Southern Hemisphere receives the majority. The lowest amount of energy and daylight that the Northern Hemisphere receives is on December 21st or 22nd, depending upon the year, while the Southern Hemisphere received the majority. The Northern Hemisphere receives the majority of energy and daylight on June 22, while the Southern Hemisphere receives the least. On March 21 and September 23, both Hemispheres get the same amount of solar energy and daylight.
THE JET STREAM
There is actually more than one jet stream! The jet stream most of us think of is the Polar Jet Stream, which recently brought us the historic and prolonged Arctic outbreak during the Christmas Holidays in 2017 into January 2018.
Jet streams are narrow bands of strong wind in the upper atmosphere. The winds blow from the west to the east because of the Earth’s rotation, while the flow shifts from north to south depending on the phase of the NAO (North Atlantic Oscillation) and other atmospheric conditions. The jet streams follow the divide of cool and warm air. These boundaries are most noticeable during the winter months of both the Northern and Southern Hemispheres.
Areas around 30°N and S and 50°N and 50°S have the largest temperature swings and the strongest winds. Due to the increase in temperature differences, the wind speed must also increase. The 50°N and S areas house the Polar Jet Streams while the 30°N and S areas house the Subtropical Jet Streams. Wind speeds within each jet can reach an upward of 275 miles per hour (239 kts or 442 km/h).
These jet streams may shift and appear differently due to multiple factors; such as where areas of high and low pressure reside, as well as warm and cool air masses, and the seasons. These jet streams may dip or rise (in altitude or latitude), they may split, or they may disappear completely and reform elsewhere.
During seasonal changes, the jet streams shift toward the poles in the summer and in the winter, the jet streams shift toward the equator.
THE HYDROLOGIC CYCLE
The Hydrologic Cycle otherwise known as the Water Cycle is the continuous circulation of water from the ground to the atmosphere and back again. There are many steps in this cycle, however, the most important and well-known are evaporation, transpiration, condensation, precipitation, and runoff.
Over land, the Water Cycle is very complicated. There are a lot of steps.
Evaporation is the change of state in something from a liquid to a gas. In atmospheric science, the main substance that evaporates is water.
Water evaporates with energy. The more energy applied to something makes the object hotter, in this case, it is liquid water. The energy being applied to the liquid water is making the molecules in the water move faster, which causes the temperature to rise, which causes water to evaporate. This energy can come from the Sun, atmosphere, the Earth’s surface.
Transpiration is a form of evaporation. It is the evaporation of water from plants through the stomata (stomata are the pores in leaves connected to the plant’s vascular tissues). Transpiration is controlled by the humidity of the air and moisture content of the soil. Only 1% of water passing through the plant is used in the growth of the plant, while the other 99% is emitted into the atmosphere.
Condensation is when water vapor in the air is changed from a gas into a liquid. In meteorology, condensation appears as clouds or dew. In everyday life, you may see it on the edge of a cold drink, or on the inside or outside of a water bottle.
It does not matter what the temperatures is for condensation to occur. It is the difference between two temperatures, which are the air temperature, and the dewpoint. The dewpoint is the temperature where dew can form. If it is cool enough, the air becomes saturated with water. If it becomes cooler, the water vapor in the air will condense into a cloud. In order for fog to occur, the temperature and the dewpoint have to be equal.
Precipitation is what happens after the condensation of water molecules become too heavy for the rising air masses to support. Therefore, the water molecules fall to Earth in the form of rain, hail, sleet, freezing rain, or snow.
Runoff is what occurs in result of excessive precipitation. Due to the excessive precipitation, the ground becomes overly saturated, which means that the ground can not absorb any more water. Rivers and lakes have been formed due to excessive runoff, which is a process that takes thousands of years. Very little evaporation occurs in a river or lake, so in order for the water to escape, it will eventually flow into the ocean, in which the water will get evaporated for the cycle to start again. In a lake, evaporation is the only way that water molecules can be returned to the atmosphere. On occasion, the water molecules may contain grains of salt. Since salt does not evaporate at the temperatures water can, the salt grains are left in a lake, which in turn may make the lake salty.
The ocean also plays a big, but simple role in the Hydrologic Cycle.
Nearly all of the water used in the Hydrologic Cycle is contained in the oceans. In fact, over 96.5% of Earth’s water is contained in the ocean. On average, about 45 inches (114 cm) of water evaporates from the ocean annually. It can take up to a thousand years for one molecule of water to move from the oceans to the atmosphere. The highest rate of evaporation occurs in winter in the Northern and Southern Hemispheres. The east coasts of North America, South America, Asia, Africa, and Australia tend to have more evaporation than the western sides. This is because winter storms (in North America, Australia, and Asia) move off of the east coasts which have stronger winds than the west coasts. These winds carry the water vapor away from its starting point. This allows more evaporation to occur.
Warm ocean currents, such as the Gulf Stream play a big role in evaporation. Warm water is carried northward by the ocean currents. Cold air masses drift over the warm water. This makes the atmosphere unstable, which causes a huge difference in ocean temperatures and air temperatures. The unstable atmosphere makes water evaporate more. The increased evaporation combined with an unstable atmosphere fuels winter storms. More powerful winter storms increase the amount of snow that the storm will produce. The excessive snowfall will eventually melt, which will increase the runoff, which causes massive flooding. However, over 90% of the evaporated water from the ocean falls back into the ocean. The other 10% falls over land
There are many things needed for precipitation to occur and form. The first and probably most important thing you need is moisture, otherwise the air is going to be dry and you will simply get nothing out of it. Most of the moisture comes from evaporated water from the Gulf of Mexico and the Atlantic and Pacific Oceans. The winds flowing around high or low pressure systems will drive the moisture inland. Clouds will then form by the lifting of the air. The air is usually lifted by being forced upward close to the fronts of low pressure areas. The air may also be forced up and over mountains.
The evaporated water in the clouds form water droplets or ice crystals (if cold enough).
Water droplets are too small and lightweight to fall to the ground as precipitation. In order for rain to occur, we go through a process called the collision and coalescence process (a.k.a. warm rain process). Collisions will occur between all of the water droplets. None of the water droplets are the same size, so their speed is different. When colliding, they fall and stick together forming larger rain drops, which is known as coalescing.
In order for rain, snow, sleet, or hail to occur, all you need are heavy ice crystals and water droplets in a cold cloud. This is known as the ice crystal process. Water vapor will drop onto the ice crystals, which causes the crystals to grow in size. Sooner or later, the ice crystals will become too heavy to be sustained in the cloud, so they will fall to the ground as snow or sleet if the Earth’s surface is cold. If the surface is well above freezing (45°F or more), the ice crystals will melt as cold rain.
- Rain | Rain is the most common type of precipitation. The droplets are 0.02 inch (0.5 mm) or more.
- Drizzle | Drizzle is kind of like rain, but the droplets are very small and they are very close together. They appear to float when following the wind or air currents. Drizzle often occurs with fog.
- Hail | Small or large ice balls that usually come from thunderstorms. Hail size tends to be about 1/4 of an inch (5 mm) to 1 inch (2.5 cm). Hail larger than 1 inch (2.5 cm) usually comes from severe thunderstorms.
- Graupel | White ice grains that look like little balls of snow. The diameter is about 1/4 inch (5 mm).
- Sleet | A form of precipitation that is nearly transparent in color and small odd shaped ice pellets.
- Freezing Rain | Cold rain that freezes when it hits the ground.
- Snow | Frozen form of rain. Snow is white and generally branches off into six-pointed stars.
- Ice Crystals | Ice crystals usually fall in cold regions such as Canada. These ice crystals are frozen particles of water, but they appear kind of like fog.
- Mist | Mist is visible water particles in the atmosphere that reduce visibility to less than 7 miles (11 km) and more than or equal to 5/8 mile (1 km). Mist and haze appear the same, but the difference lies with the difference between the air temperature and the dewpoint. If the difference between the air temperature and the dewpoint is 3°F (1.7°C) or less, then we define the obscuration as mist.
- Fog | Fog is a type of cloud that made up of visible water droplets near the Earth’s surface that reduce visibility to less than 5/8 mile (1 km). Fog is like drizzle, but instead of falling to the ground, it just floats in the air.
- Smoke | Smoke is made up of small particles that are suspended in the air. Smoke is created from combustion. If the particles travel between 25 and 100 miles (40 to 160 km) or more, the smoke may become haze.
- Haze | Haze is the suspension of very small dry particles that are invisible to the human eye. Due to the numerous amount of these particles, the cluster becomes visible to the naked eye. Haze and mist look alike. If the air temperature and the dewpoint are greater than 3°F (1.7°C), then the obscuration is called haze, NOT mist.
- Volcanic Ash | Volcanic ash comes from erupting volcanoes. It contains rock powder that may get trapped in the atmosphere for a long time before falling back to Earth.
- Dust | Dust is made up of small particles of dirt, sand, rock, or other Earth matter that when carried by strong winds, may reduce visibility. This occurrence is known as a dust storm.
- Sand | Sand, when carried by wind, may reduce visibility. This occurrence is called a sandstorm.
- Sand/Dust Swirls | A swirling, vertical column of sand or dust that kind of resembles a tornado.
- Squall | A squall is strong wind that suddenly occurs. Wind speed must increase at least 18 miles per hour (16 knots or 30 km/h) and blow for 25 miles per hour (22 knots or 41 km/h) or more for at least a minute. These sudden winds usually occur along thunderstorms (term ‘squall line’). These squalls may also occur with passing snow showers, which is called a snow squall.
- Tornado | A tornado is a rotating column of air often containing debris. A funnel cloud is basically the same thing, but the column does not touch the ground.
- Waterspout | A waterspout is a tornado over water. It is a rotating column of air over the water.
Clouds are visible minute water or ice particles in the atmosphere. They can weigh up to tens of millions of tons and be carried by winds of 150 miles per hour (240 km/h) or stay still as the wind passes through the cloud. Clouds usually cool the air, but they may also keep warm air near the surface.
So how do clouds form? Clouds need two things to form; water vapor and nuclei.
Water molecules in the atmosphere are too little to bond together, so they need flatter surface objects with a radius of at least one micrometer to form a bond. These ‘objects’ are called nuclei. These nuclei are solid and liquid particles suspended in the atmosphere. Some of these particles are from fires, volcanoes, ocean spray or wind-blown soil.
However, these particles and water droplets can’t bond together without a saturation point. The temperature must be equal to the dew point (saturation point) for evaporation to equal condensation. Clouds will form when a block of air called a parcel has water vapor; this block of air has to cool below the dew point. The easiest way for this to occur is for the air to lift from the surface into the atmosphere. As the air increases in altitude, the air moves into an area of low pressure. This allows the block of air to grow in size. As it expands in size, the heat energy gets removed from the parcel, which results in the cooling of the parcel, which forms a cloud. This is known as the adiabatic process. These parcels cool at different rates as they rise. The rate at which the parcel cools as it rises is referred to as the lapse rate. For example, if the air has a relative humidity <100%, the parcel will cool 5.5°F every 1,000 feet (9.8°C per km). If the relative humidity reaches 100%, then excess water vapor will condense onto the cloud resulting in the formation of a cloud droplet. Once the block of air (parcel) reaches the saturation point, which is when relative humidity is at 100%, vaporized water will condense onto the cloud’s nuclei in the form of a cloud droplet. Because the atmosphere is always changing, the rising air combines with dry air. This allows condensation and evaporation to continuously occur. With all of this occurring, clouds are continuously able to change shape and appear or disappear.
There are many different sizes and shapes of clouds, but there are four basic forms of clouds; cirro-form, cumulo-from, strato-form, and nimbo-form.
- Cirro-form | Cirro-form clouds are white and they look like strands of hair; as the Latin word for curl of hair is ‘cirro.’ Cirro-form clouds are made up of ice crystals, and they are high in the sky. They first appear before a low pressure area such as a thunderstorm, supercell, or tropical storm system.
- Cumulo-form | Cumulo-form clouds look like white cotton balls. They are tall vertically, in which they show thermal uplift of air in the atmosphere.
- Strato-form | Strato-form clouds are like a big gray blanket over the sky. These clouds are very wide and they form from non-conductive rising air. Strato-form clouds usually form to the north of warm-fronts.
- Nimbo-form | Nimbo-form clouds are rain clouds. Since almost all precipitation occurs from these clouds, nimbo-form clouds have the largest vertical height.
THE HEIGHT OF CLOUDS
The divide between the polar and temperate regions is the Arctic Circle at 66.5°N in the Northern Hemisphere and at the Antarctic Circle at 66.5°S in the Southern Hemisphere. The divide between the temperature and tropical regions are the Tropics of Cancer 23.5°N and the Tropics of Capricorn at 23.5°S. However, the division between these regions vary from day to day, day to night, from season to season, and from year to year. The jet streams divide these layers with the Polar Jet Streams dividing the polar and temperate regions and the Subtropical Jet Streams dividing the temperate and tropical regions.
Because the jet streams are strong winds, the flow makes the maximum altitude of the tropopause decrease in each region when you move from the equator to either pole. As the tropopause decreases in height, so do the elevations at which clouds form. However, low clouds are the only exception, because they generally form between the Earth’s surface and 6,500 feet above.
Cumulus and cumulonimbus cloud bases can sometimes be higher than 6,500 feet (2,000 m). In the summer, the bases or bottom of these clouds reach into the mid-level cloud range in non-mountainous areas in the southwest part of the United States. In north and central Texas, cumulus cloud bases have been at a staggering 9,000 feet (2,750 m). In southern areas of Texas, cumulonimbus clouds have been seen at 11,000 and 12,000 feet (3,350 and 3,650 m). The reason for the cumulus and cumulonimbus cloud formations at such high levels in the more temperate to tropical regions is because the parcels of air need to rise to about two miles (3 km) above Earth’s surface to cool down and reach the point of condensation. The lower levels of the atmosphere are dry, while the mid-atmosphere tends to have more unstable air with dry and moist areas. Because it is so dry in the lower atmosphere, the parcels have to rise that far before condensing into clouds.
THE TYPES OF CLOUDS
Cloud color generally depends upon the color of light the cloud receives. The main source for this light is the Sun. The Sun gives off white light, which is a combination of all of the visible colors. Every visible color represents electromagnetic waves with different wavelengths. These wavelengths will increase from violet to indigo to blue, then green, yellow, to orange, to red and dark red. As the wavelength increases, the light’s energy decreases. With the use of a prism, you can see how the higher-energy light waves refract the most, while the lesser energy light waves refract the least. The velocity of light decreases slightly when it shines through a prism, which causes the light to bend. Rainbows are caused by this, but instead of a prism, it is a raindrop.
The reason cloud color is generally white is because each water molecule that forms a cloud is relatively the same size as one wavelength of sunlight. Because they are relatively the same size, a type of distribution occurs, called Mie Scattering. Mie Scattering distributes all of the wavelength colors at once. Because the light waves get scattered all at once and get equally distributed, the cloud color we see is white.
Once in a while, you will see clouds that appear orange, yellow, or red. These brighter colors can be attributed to mist, haze, or dust in the atmosphere as well as a combination between Rayleigh and Mie Scattering. These colors are generally expressed in clouds during the morning and evening hours. The color of the cloud(s) are due to Rayleigh Scattering and the color of the sunlight we see is due to Mie Scattering. When sunlight enters the atmosphere, the first light waves to spread are the blue ones. This means that the yellow, orange, and red light waves are able to go further, in which the color of the cloud is seen as being yellow, orange, or red. The light gets refracted when it gets close to Earth’s surface. This allows the path the light goes to become longer, which allows more Rayleigh Scattering to occur. The yellow light waves will be the first to be distributed, the orange second, which will leave the red wavelengths alone to be expressed.
Sometimes, clouds will appear gray because they become so heavy, that sunlight gets blocked out. Clouds may also appear bluish-grayish if there is a lack of sunlight and the cloud is thick or if the cloud is semi-transparent and the blue sky is blended in with the cloud.
Sky color, however, is generally blue because of scattered light waves from the Sun, especially the ones with short wavelengths. Unlike clouds, the atoms and molecules which make up the gasses in the atmosphere are very small and they are much smaller than the wavelengths of the light from the Sun. When the Sun’s light waves penetrate the atmosphere, they move around by colliding with the atmospheric molecules. This is called Rayleigh Scattering. As the light waves scatter, the violet light waves are the first to spread. These violet light waves get distributed high in the atmosphere, which is why when you get closer to outer space, the sky appears really dark purple in color. In the middle regions of the atmosphere, the sky appears indigo when you are in an airplane because that is where the indigo light waves get distributed. Lower in the atmosphere is where the blue light waves scatter. These blue light waves are mixed with some violet and indigo light waves, but because there are much more blue light waves than there are violet and indigo, the sky appears light blue in color.
There are approximately 14.6 million thunderstorms each year across the globe, which is about 40,000 storms daily. The United States has a lot too no doubt, especially in the Midwest, Plains, and Southeast. Florida has the most number of thunderstorms each year; 80 to 100 or more annually. The southeastern states as a whole have the most thunderstorms in the United States because of the warm, moist air from the Gulf of Mexico and the Atlantic; which is vital in thunderstorm development.
INGREDIENTS FOR A THUNDERSTORM
In order for thunderstorms to develop, you need three main things: moisture, atmospheric instability, and a lifting mechanism.
Most of the moisture for thunderstorm development comes from the oceans, however, some comes from rivers, lakes, bays, and in the United States, the Gulf of Mexico. The water temperature is one of the biggest factors in determining how much water evaporates. Generally, the warmer the water, the more water will evaporate. The cooler the water, the less will evaporate. This is why El Nino winters tend to be snowier and wetter than La Nina winters.
Ocean currents also have large influences on thunderstorm development and moisture content in the atmosphere. As you will see later, in the ocean lesson of this website, ocean currents on the western side of continents are cool currents, and ocean currents on the eastern side of continents are warm currents. As we mentioned above, evaporation rates are higher in warmer water, therefore evaporation is higher in warmer ocean currents than cool currents. Because the Gulf of Mexico and the Western Atlantic Ocean are in warmer ocean currents, this means that the evaporation there is higher than the rates are in the Eastern Pacific, along the west coast. The southeastern states of the U.S. are along the Gulf and Atlantic, which is why they are prone to the most annual number of thunderstorms, unlike southern California, which is along the California Cool Current, and has few thunderstorms and is often drier.
When air is characterized as being unstable, that means that the air is continuously rising or continuously sinking. An air mass is considered unstable when you got dry air atop the moist air at the bottom. When this kind of parcel lifts, it will cool and condense into a towering cumulonimbus cloud, which is the “notorious” thunderstorm cloud.
Lifting is necessary for thunderstorms to develop. There needs to be a mechanism which starts the upward motion. Some of the Sun’s energy gets into the air, which results in different air densities. If the air is denser, it will sink; if the air is less dense, it will have the tendency to rise. There are many other factors that come into play when it comes to lifting.
One of these factors is differential heating. The Earth’s surface is different in various locations. Some surfaces heat faster than others. Pavement heats faster than a grassy surface, and a grassy surface will heat faster than water. Water has a very high specific heat, which means that it takes a lot of energy to increase the water’s temperature by one degree Celsius. Paved surfaces have a very low specific heat, which means that just a little bit of exposure to a heat source will increase the temperature of that surface. Grass is in the middle of the two; it requires more energy than the pavement to heat the surface, but it requires less energy than water.
Specific heat is the amount of heat energy per unit mass required to increase the temperature by one degree Celsius. In our case, water’s specific heat is 1 calorie/gram °C, which is higher than any other common substance, while grass’s specific heat is around 0.24 calorie/gram °C. Pavement’s specific heat is 0.22 calorie/gram °C and concrete’s is 0.21 calorie/gram °C. The Earth is mostly made up of water, with 71% of its surface being covered in oceans. This is one of the reasons that Earth’s temperature is a comfortable 59°F (15°C) on average. If instead, land covered 71% of Earth’s surface, then the global average temperature would likely increase by 10% to 30% of what it is currently.
Due to the difference in heating on different surfaces, the air will also have different densities in adjacent areas. As we will discuss later, the cool air will sink, presumably due to the force of gravity, and the less dense warm air will rise. Terrain also plays a role in this. When a parcel of air reaches a mountain, it will be forced up because of the terrain. This is common in the Rockies, where they have what is known as upslope thunderstorms.
Fronts, drylines, and outflow boundaries each play vital roles in thunderstorm formation. A front represents the boundary between warm and cool air masses. The cooler air will lift the warm air suddenly. If the air in the warm air mass has enough moisture in it, a thunderstorm will develop along the cold front.
Drylines act in a way similar to fronts. Drylines represent the divide between air masses with different moisture content; they divide warm, moist air from dry, hot air. Moist air is less dense than dry air, in which the moist, less dense air gets lifted over the denser, drier air. The air temperature behind these drylines are higher because there is a lack of moisture in the air. The air in front of the dryline is less dense, allowing it to lift, forming thunderstorms. These drylines are most commonly found across the Great Plains during the spring and early summer.
Outflow boundaries are “mini cold fronts” that are formed due to a sudden dart of cooler air as a thunderstorm passes aloft. Like both fronts and drylines, warm, moist air will lift above the cooler and drier air, which will allow more storms to form afterwards.
A thunderstorm tends to go through its life cycle in about 30 minutes time.
The first stage is the Towering Cumulus Stage, which is when the air inside the cumulus cloud starts to flow upward, which known as an updraft. This causes the cloud to grow in height to a about 20,000 feet (6 km). The cloud may eventually reach 60,000 feet (18 km) in height.
The second stage of a thunderstorm is the Mature Cumulus Stage, which is when the cloud becomes a cumulonimbus cloud and reaches a height of somewhere between 40,000 and 60,000 feet (12 to 18 km). This is the dangerous stage of the storm, which consists of both updrafts and downdrafts. This is a recipe for disaster, as this is when tornadoes, hail, flash flooding, and damaging winds may occur.
The final stage is the Dissipating Stage, which is when the downdraft cuts off the updraft, which in turn cuts off the supply of moisture and energy to the cloud, which causes the storm to start to stop. Rain and light wind may continue for a little while after as the cooler air flows underneath the warmer, less dense air, but it won’t be long before all of it dissipates.
- Ordinary Cell | These storms are short because there is only one cell; one updraft and one downdraft. During the towering cumulus stage, the air current flowing upward, known as the updraft suspends raindrops in the air. Eventually, these raindrops will become too heavy which causes them to fall, which leads to the formation of the downdraft. The downdraft will cut off the supply of moisture by cutting off the flow of the updraft, which ends the lifespan of the storm fairly quickly. Hail and wind are indeed possible in these storms, but these are uncommon, and even if they do develop, they probably won’t do much damage. If conditions are right, another cell could form, which may result in an EF1 or EF2 tornado and/or microbursts.
- Multi-Cell Cluster | Thunderstorms that generally develop in clusters are referred to as a multi-cell cluster. Each cell in the cluster are in different stages of their life cycle, in which they all merge together into one big storm. Each cell acts on its own, but as the first cell matures, it gets carried downstream by upper level winds, which allows new cells to form behind the preceding one. The speed at which the multi-cell cluster moves makes a big difference in the amount of rainfall a particular area gets. Usually, the first cell will move downstream and the succeeding ones will move in the same path as the original. These patterns tend to produce astronomical rainfall totals in a small area, which leads to flash flooding. Storms like this are known as ‘training echoes’ and are easily identified on the doppler radar by seeing how each cell follows the path of the one in front of it. Every so often, if the conditions are right, the storm may appear as if it is moving backwards against the wind, but they are just back-building. Back-building thunderstorms will also dump a lot of rain over a small area.
- Multi-Cell (Squall) Line | Every now and then, thunderstorms will develop in a line that may be local or extend for hundreds of miles. Squall lines may last for hours upon hours and could produce damaging winds and hail. In these storms, new cells and shelf clouds are constantly forming at the leading edge of the system with the heavy precipitation following behind. Within these cells are individual updrafts and downdrafts that can be very strong, which may result in large hailstones and very powerful outflow winds. The shelf clouds are the clouds that look like a shelf and are a result of rain cooled air getting distributed underneath the squall line; this acts like a mini cold front in which the cooler air forces the warm air to rise, in which the warm air eventually condenses forming the shelf cloud in the process. Tornadoes may be spawned on the leading edge, but they usually don’t occur as squall lines tend to only produce straight-line winds, which cause a lot of damage to forests and structures in its path. Derechos may also form from squall lines, but they usually do not.
- Supercells | Supercells are a unique type of organized single-cell thunderstorm; they can persist for many hours and are known to spawn tornadoes, large hail, strong winds, and downpours of rain, which leads to flash flooding. They consist of updrafts flowing over 100 miles an hour (160 km/h), which are responsible for large hail and tornado outbreaks. The downdrafts of the storm are responsible for producing outflow winds and downbursts of over 100 miles per hour (160 km/h). The reason that a supercell forms is because of vertical wind shear. Wind shear is when winds change direction with height. In our case, the winds will abruptly change direction OR start turning clockwise with height. Each supercell shares similar characteristics, but each one is unique, which is why we categorize then into three groups based on the potential rainfall the storm holds. We have the Rear Flank Supercell, the Classic Supercell, and the Front Flank Supercell. The Rear Flank has low precipitation, while the Classic has average precipitation, and the Front Flank has high precipitation. In a Rear Flank Supercell, the updraft is located on the rear flank of the storm, which gives the system a corkscrew-like appearance. Precipitation from these storms is often very little and is often removed from the updraft of the cell. The Classic Supercell has a large, flat updraft base on the front flank of the storm, with bands around the updraft. These supercells are accompanied with very high precipitation alongside the updraft. Hail and long-lived tornadoes are also often associated with the Classic Supercell. Underneath the storm, the rotation is often visible, with a wall cloud forming below the rain-free base or below the storm’s updraft on the rear flank of the storm. Wall clouds may be obscured by surrounding rain. If a supercell spawns a tornado, the tornado will usually form within the wall cloud, making it challenging to see.
THUNDERSTORM HAZARDS: HAIL
Hail is a form of precipitation that is in the form of small or large ice balls that usually come from thunderstorms. Hail size tends to be about 1/4 of an inch (5 mm) to 1 inch (2.5 cm). Hailstones larger than 1 inch (2.5 cm) come from severe thunderstorms. Hail can damage property, including aircraft, homes, cars, and outboard vehicles, as well as posing a threat to life. Hail forms when updrafts carry raindrops up into very cold areas in the atmosphere. Hail grows in size as it reaches the extremely cold areas by collision with supercooled water droplets in the updraft. Two methods exist in which hailstones will grow; wet growth and dry growth.
In wet growth, a small piece of ice known as the hailstone nucleus is in an area of the atmosphere with temperatures slightly below freezing. When the water molecules in the updraft collide with the hailstone nucleus, the water adheres to the ice pellet, but it does not freeze immediately. Rather, the water slowly freezes, and because the process is rather slow, air bubbles can escape which results in a clear layer of ice.
In dry growth, the air temperature will stay well below freezing, which means the water droplets will collide and freeze immediately after contact with the hailstone nucleus. The air bubbles are frozen in place, which leaves the hailstone “cloudy.”
Powerful updrafts form a rain-free area in the supercell or thunderstorm, which is called the “weak echo region.” The area is bounded on one side and bounded aloft by very heavy precipitation which is seen on the radar as a strong echo. The upward motion of the updraft is what suspends the precipitation above. The hail nucleus levitates and gets carried aloft because of the flow of the updraft. As it floats, it grows in size as it collides with supercooled raindrops and it will eventually fall to the Earth or get blown out of the air current resulting it to still fall to Earth. Other times, the hail nucleus could be so light, or the updraft could be so strong, that the nucleus gets blown back into a cloud, where the hailstone nucleus will collide with more supercooled raindrops. This process can repeat multiple times, depending on how heavy the hailstone gets. Usually, the stronger the updraft, the larger the hail will get.
Multi-cell storms can produce dozens of individual hail storms, but generally, the hailstones associated with these storms are relatively small. The reason for this is because the mature stage in the life cycle of these multi-cell thunderstorms are short, in which the updrafts can not become strong enough to produce large hailstones and decreases the available time for hailstone growth.
However, in a supercell, the updrafts are sustained and constant, in which the hailstone nucleus gets repeatedly tossed into the top of the cloud in which it collides with supercooled raindrops and eventually falls to Earth after becoming too heavy for the updraft to sustain it.
Different speeds of the updraft form different hail sizes. The table below has data from the National Weather Service, which shows you the average speed for each individual size.
It is interesting to note that while Florida has the most thunderstorms per year, Texas, New Mexico, Oklahoma, Kansas, Nebraska, and Colorado have the most hailstorms. The reason Florida doesn’t see many hail storms is because the freezing level is so high, that the hail will usually melt before reaching the ground.
THUNDERSTORM HAZARDS: FLASH FLOODING
Aside from heat or cold related deaths, flash flooding is the main weather-related cause of death. The reason for this is because most people who become injured don’t tend to have common sense when driving or walking through water; they underestimate the power of water. It only takes six inches of rushing water to knock you down. On a yearly basis, an average of 127 deaths occur each year. Now compare that to 73 for lightning, 68 for tornadoes, and 16 for hurricanes. Most fatalities from flash floods are vehicle-related, in which the driver underestimates the power of fast-moving water.
Typically, a flash flood occurs due to a slow moving thunderstorm, or thunderstorms that move over the same area over and over again, or from tropical storms and hurricanes. However, flash flooding has also been known to occur from Front Flank Supercells and Multi-Cell Cluster storms. How fast it takes for these flooding events to occur depends upon how intense the rain is or how long the rain falls, topography, soil conditions, and ground cover.
Flash floods can cause bridges to collapse, destroy roads, buildings, knock down trees and power lines, roll boulders, or cause mudslides. If rain causes the water levels in a river to rise suddenly, the river may rise as much as 30 feet (m) or more.
You can stay safe when there is a flash flood by following these rules from the National Weather Service. TURN AROUND DON’T DROWN®.
THUNDERSTORM HAZARDS: DAMAGING WIND
Damaging wind is by far more common than tornadoes during a thunderstorm. On occasion, these damaging winds will blow in a straight direction, known as straight-line wind. These straight-line winds can blow a whole section of forest down like a set of dominoes. When this occurs, people often rely on weather superstition and blame it on tornadoes.
Damaging winds are formed by a downdraft. When air rises, it will cool to the condensation point in which water vapor condenses into water droplets, forming a cumulus cloud. Somewhere near the center of the updraft, the particles collide and coalescence which results in larger water droplets. This process will continue until the water drops become too large to be supported by the updraft. When the raindrops fall, friction causes the updraft current to sink and fall downwards, forming a downdraft, which stops the convection of the storm, which eventually causes the storm to dissipate.
While this occurs, evaporation will force heat energy to be removed from the atmosphere, causing SOME of the falling water drops to evaporate and cooling the air. The cooling of the downdraft allows the downward flow of the air to become denser, which forces the air to sink even faster. Once the air approaches the Earth’s surface, it spreads with the leading edge of the cool air creating a gust front. This causes the temperature to fall rapidly and the wind speed to increase suddenly up to 60 miles per hour (96.5 km/h) or more. A gust front may act as a lifting mechanism to form more thunderstorms, or it may cut off the supply of moisture for developed cells.
Damaging wind comes in different forms.
- Downbursts | Downbursts are a form of damaging wind that are produced by a downdraft over a horizontal area extending up to six miles (10 km). In wet areas, the downburst will be accompanied with extreme rainfall and in dry environments, these are known as dry downbursts. In deserts, downbursts may produce dust storms.
– Microbursts are a form of downbursts that are small and don’t spread far horizontally. They tend to only go about 2.5 miles (4 km) or less. But don’t underestimate these small downbursts, as they can produce wind gusts up to 168 miles per hour (270 km/h). Microbursts only last between two and five minutes before dissipating, but despite being short and sweet, the strong gusts can pose problems for aircraft.
– Macrobursts are another form of downbursts. They are larger than microbursts and they can extend horizontally for more than 2.5 miles (4 km) and can produce wind gusts up to 130 miles per hour (210 km/h). Macrobursts can last as long as 20 minutes before dissipating.
- Heat Bursts | These are an extremely rare form of dry downbursts that generally occur with dissipating storms. Heat bursts typically occur during the late evening or night, and may cause damaging winds and cause a sudden spike in temperature alongside a sharp drop in the humidity levels dewpoint. The process begins high in the atmosphere; a pocket of cool air forms above due to the evaporation of the falling raindrops in the downdraft. As in every other downburst, evaporation of the water drops will force heat energy to be removed from the atmosphere, but unlike the typical downburst, the evaporation will cause ALL (NOT some) of the falling water drops to evaporate and cooling the air. As the cooler air sinks, it will compress against the surface forming a high pressure area, causing the air temperatures at the surface to spike up suddenly. The reason the cooler air doesn’t cause a sudden drop in temperature in heat bursts Is because there are no raindrops to absorb the heat. Temperatures usually rise somewhere between 10 and 20 degrees Fahrenheit in a matter of minutes and they can stay that way for a few hours before cooling. The most widely known heat burst occurred near, in, and around Wichita, Kansas on June 9, 2011. Temperatures climbed from the low and mid 80s to as high as 102°F (39°C).
THUNDERSTORM HAZARDS: TORNADOES
Many probably don’t realize, but tornadoes are violent rotating columns of air that touch the ground and are associated with severe thunderstorms and supercells. A tornado typically rotates counterclockwise in the Northern Hemisphere, like hurricanes. The life span of a tornado is typically a few minutes, but sometimes, they can last as short as a few seconds, or as long as one hour. On extremely rare occasions, there have been tornadoes that last as long as two hours, but there was one in 1925 that lasted three hours and 29 minutes. Despite having a short life cycle, they can cause widespread damage while travelling many miles.
It is really unknown how many tornadoes strike the U.S. each year. There is so much year to year variation that tornado counts for any given year are unjustified. They can range from 250 to 1,000 or more. Tornado reports have increased a lot since 1950, but that is because of an increase in population and technology over time. However, the actual number of tornadoes have had almost no trend. What is known is that the most tornadoes in the world take place in the Central U.S., which is dubbed “tornado alley.”
Although it is unknown how many tornadoes occurred each year prior to 1950, we do know that the 1930s had the worst tornado outbreaks in history and that 1936 was the worst season to date. In fact, 1936 was a crappy year altogether. February 1936 was one of the coldest on record for the Lower 48; temperatures plunged as low as -60°F (-51°C) in North Dakota and the Blizzard of ’36 left the East Coast paralyzed in a foot of snow. By March, the snow had melted as the eastern 2/3 of the U.S. went through a massive thaw with the whole month averaging four to five degrees above average. The snow-melt and excessive rain caused widespread flooding from the Mississippi to Washington D.C. to New York and Pittsburgh. The spring and early summer brought on deadly tornado outbreaks across the Plains and Midwest and the summer of 1936 was the hottest on record in the U.S. North Dakota reached an all time record high of 121°F (49°C) and Omaha, Nebraska had a night where the minimum temperature didn’t fall below 93°F (33.9°C).
So how do tornadoes form? Well, generally, tornadoes will form from supercell thunderstorms, which are rotating updrafts of air. Supercells develop in places with a strong vertical wind shear, which is the change in wind speed and/or direction with height. The image below depicts wind blowing from southeast to northwest. As elevation increases, the wind direction changes in a clockwise motion; south, then southwest, then west, and so forth.
Speed shear is the change in wind speed with height. Speed shear creates a rolling effect in the atmosphere, which is vital in mesocyclone development, which may spawn tornadoes.
Strong vertical shear is a combination of the two. This is the most common atmospheric condition for supercell formation.
The updraft will lift the rotating air that is formed by the wind shear. This results in two circulations within the supercell; a clockwise rotation, and a counterclockwise rotation. The directional wind shear is associated with the counterclockwise rotation, as it enhances it, but it diminishes the clockwise rotation. This results in the counterclockwise rotation to remain, which is known as a mesocyclone, or supercell. When you view this from the radar, it looks like a hook, because the air rises within the storm, which results in the storm becoming “stretched” and narrower.
It is currently unknown what causes the funnel of a tornado to form. Some suggest that within a mesocyclone, tornado formation can be attributed to the differences in temperature along the edge of the downdraft. However, other scientists suggest that based off mathematical modeling studies, tornadoes can develop without the temperature differences.
The funnel cloud of a tornado is made up of moist air. When a funnel cloud descends, water vapor inside the funnel condenses into water droplets, which are the same as cloud droplets, but they are not part of the cloud because they form within the funnel itself. The funnel cloud is only visible because the water droplets inside the funnel are spread out against the boundary of the funnel. The funnel appears white because of the water droplets inside the funnel. Dust and debris will often begin to rotate due to the strong circulation of air aloft, as the funnel descends down. When the funnel reaches the ground, it becomes a tornado. The color of the tornado will often change due to the color of dust, dirt, and debris on the ground; sometimes the tornado will appear red, black, brown, or gray.
Tornadoes are not measured by their lifespan or size. As we discussed above, some tornadoes last only a few seconds, while others can last upwards of an hour or more, but most generally last less than ten minutes. On the other hand, small tornadoes have been known to occasionally produce considerable damage, while large and wide tornadoes have been known to occasionally produce minor damage. A tornado will slowly lose its strength over time, as the funnel will decrease in size, and the tornado will become tilted with height. This eventually results in it getting a rope-like appearance as it dissipates.
Tornadoes, like hurricanes, are classified by their wind speed. To determine the intensity of a tornado, we use the EF Scale. The EF (Enhanced Fujita) Scale was developed by a group of well known meteorologists and wind engineers in 1971. The EF Scale was revised from the original F (Fujita) Scale, which was developed by Dr. Tetsuya Fujita to estimate tornado wind speed based off damage caused by the tornado.
The original F Scale was limited. For one, there was a lack of tornado damage indicators and no account for the quality of a structure’s construction or structure. For two, there was no correlation or proportion set up between tornado damage and wind speed. This likely resulted in many tornadoes prior to 1971 being overestimated or underestimated in strength. The revised EF Scale takes more factors into consideration. The EF Scale has 28 damage indicators, which include building type, structures, and trees. Each damage indicator has eight degrees of damage. These can range from minor visible damage to total destruction of a structure or building.
Derechos are very large, long-lived windstorms associated with bands of fast-moving thunderstorms that form bow echoes. The damage pattern associated with these storms is often a straight line due to the straight-line winds produced by the storm.
The main cause for these storms are a family of downburst clusters. These clusters extend for 50 to 60 miles (80 to 100 km). Downbursts are a form of damaging wind that are produced by a downdraft over a horizontal area. Within these downbursts, are microbursts, which are smaller pockets of very intense wind. Within the microbursts are even smaller pockets of powerful wind called burst swaths; they can range from 50 to 100 yards (45 to 140 m). The damage pattern often resembles that of a tornado. Due to the setup of the storm, damage can vary from place to place. Within areas of damage, there can be areas within the overall area that suffered more damage than others. This is the reason that any given house or building can be completely destroyed while the one next to it can be nearly intact.
You can stay safe from derechos by following these guidelines from the National Weather Service.
As we mentioned above, derechoes are often associated with fast moving thundershowers or storms that take a curved shape that resemble a bow. The term bow echo is based on how bands of rain “bow out” when strong winds reach the surface and spread out horizontally.
These bow echoes usually come from a cluster of thunderstorms, but they may also come from a single supercell. The downdraft, having been cooled by the rain reaches the Earth’s surface, it spreads out. This marks the beginning for that particular cell’s dissipating stage. The cooler air will force moist, warm air upward causing a boundary to form between the cool and warm air known as the gust front.
Once the warm air is forced up by the gust front, the next cell will form. The rain that the maturing cell produces reforms the cool air, which will sustain the gust front. As the pocket of cool air increases in size, it causes an inflow of air on the trailing edge of the storm.
This will allow the updraft of the system to become tilted, which allows the thunderstorm cloud to grow even larger in size, which increases the potential rainfall area, which results in more cooler air underneath the thunderstorm. This strengthens the gust front allowing it to bow out. The process repeats as the gust front forces more warm air upward.
The extra rain underneath the complex reinforces the cooler pocket of air and strengthens the rear inflow of air. This allows the thunderstorm to reach a semi-steady state. This also means that the storm has a bow-like appearance on the radar, with intense rainfall near the center of the cool air underneath the storm.
This process will continue to occur as long as new cells continue to from atop the gust front as it moves forward. The cool pool or pocket of air will continue to increase the rear inflow of air as the system moves, replacing dissipating or dead cells. Along the leading edge of the system, there could be thunderstorms producing microbursts and downbursts.
If the bow echo travels more than 250 miles (402.3 km) with wind gusts over 58 miles per hour (93 km/h), then the storm is called a derecho.
There are three types of derechos based on the structure of a thunderstorm and its development in forming the derecho. These three types are serial, progressive, and hybrid.
DERECHO TYPES: SERIAL DERECHO
Serial derechos are the most common to come across during the springtime and autumn. These derechos are formed by many bow echoes bunches together in one long squall line, that is generally hundreds of miles long. They tend to form with a strong upper level trough along with a strong surface low.
DERECHO TYPES: PROGRESSIVE DERECHO
Progressive derechos can travel for hundreds of miles, and typically form during the summertime and develop in a short line of storms between 40 and 250 miles (65 to 400 km) in length, alongside a stationary front where the upper level wind is parallel to the front. In other instances, progressive derechos can begin with a single bow echo, in which, over time, they can grow to being over 250 miles (400 km) in width.
DERECHO TYPES: HYBRID DERECHO
Hybrid derechos are simply a combination of progressive and serial derechos. They tend to have strong surface low pressure systems, similar to a serial derecho, but they tend to share similar characteristics with progressive derechos.
(BONUS) DERECHO TYPES: LOW DEW POINT DERECHOS
These derechos are a form of serial derechos, bu they are unusual, as they do not occur with very moist air in place. These derechos tend to form from windstorms and in areas that typically do not have much moisture in the air. When low dew point derechos occur, it is typically during late autumn and early spring alongside low pressure systems.
WHEN AND WHERE DO DERECHOS HAPPEN?
Just as with any weather event occurring, the reason for derecho formation and the way they behave are mainly due to atmospheric reasons.
The time of year and the weather pattern that is occurring at a given point in time makes all the difference in what kind of derecho forms; whether that be serial, progressive, or hybrid. 70% of all derechos occur during warm season, which is May through August. September through April is the cool season, where the remaining 30% occur. Derechos usually form in groups, which means that over a period of days, more than one derecho may occur, just not always in the same location.
Progressive derechos, as mentioned above, typically occur during the summer months. They are also usually found to the north of high pressure ridges (below). As the jet stream “follows the Sun,” so do the derechos. As we transition into the Northern Hemisphere summer, the storm tracks shift poleward with the jet stream. At the same time, high pressure areas also move northward. With that said, that puts the Midwest and Northern Plains in the target spot for the most derechos each year.
Serial derechos generally occur in the cool season, but they have been known to occur during the warm season as well. They generally occur between a trough and a ridge (above).
During the warm season in the United States, derechos form along two axes. The first extends across the corn belt; from the upper Mississippi Valley to the Ohio Valley. The other is along the southern Plains into the mid Mississippi Valley. During the cool season, derechos tend to occur across Texas and the Southeast. At other times, derechos may occur across areas in the western U.S. during the spring and early summer.
Derechos have been thought to occur in other areas besides the United States, but these events are rarely seen outside of the country. A serial derecho occurred in Germany in July of 2002, but that is the only time one such event has been documented to occur outside of the U.S. However, in parts of India and Bangladesh, there are storms that they have every spring called a Nor’wester, which have been debated as to whether they are derechos or not.
Lightning is perhaps one of the most interesting phenomena in nature. It is also one of the oldest observed natural phenomena, but it is one of the least understood, which makes it more fascinating. The main thing that everyone knows about lightning is that it is a large spark of static electricity, similar to the static electricity you feel when you get shocked by a door knob or blanket. However, getting struck by lightning probably won’t feel like a little friendly zap. Rather, it will hurt like a “bleep” and you will light up like a firework on the Fourth of July, or you might become electrified toast. Either way you look at it, there is no “bright side” to getting struck by lightning – well the lightning is bright. A lightning bolt can be as hot as 50,000°F (27,760°C).
Scientists are having a hard time trying to figure out how lightning works, and how it interacts with solar flares reaching the upper atmosphere or Earth’s electromagnetic field. Lightning is most often associated with thunderstorms (hence its name), but it has also been observed in numerous volcanic eruptions, very intense forest fires, surface nuclear detonations, snowstorms, and in hurricanes.
There are about 40,000 thunderstorms each day across the globe, which equals about 14.6 million each year. At any given time, there could be 2,000 going on at once. NASA‘s satellite research shows that thunderstorms produce lightning strikes 40 times a second across the globe.
There is widespread debate as to why and how lightning forms, but there are general concepts and conditions we know that produce lightning. The most widely accepted theory is that lightning forms due to separations of an electric charge and formation of an electric field inside a thunderstorm. Other recent theories suggest that lightning formation is also has to do with ice, hail, and graupel.
Thunderstorms are very turbulent environments. Updrafts within the storm carry liquid water droplets somewhere between 35,000 and 70,000 feet (10,668 to 21,336 m) above Earth’s surface. This is well above the freezing level. The downdrafts within the storm carry hail and ice from the upper areas of the storm. When the hail and/or ice collide with the water droplets, they freeze and release heat. The heat keeps the hail surface slightly warmer than the surrounding environment. This is what causes graupel or soft hail to form. When these soft hailstones or graupel collide with more water droplets, electrons are sheared off the ascending particles and they collect on the descending particles. Since electrons carry a negative charge, the cumulonimbus cloud will have a negatively charged base and a positively charged top.
Within the cloud itself, the positive and negative charges separate, as an electric field is formed between the top and bottom of the cloud. The charges will continue to separate resulting in a strengthened electric field. However, the atmosphere is an insulator that stops the electric flow, which means that a large amount of electric charge needs to build up before lightning can strike. Once the charge is built up so much that it overpowers the atmosphere’s insulation, lighting will form and strike.
On the ground, a positive charge forms, which results in cloud-to-ground lightning as the charge will follow the storm’s path. The positive charges at the surface tend to “climb up” tall objects, such as a tree or house. As in physics, for every action, there’s an equal and opposite reaction. In our case, the reaction is that the negatively charged particles will start to descend rapidly, faster than you can blink your eye. The positively charged particles will rise a bit slower. When the two ends meet, there is an electrical transfer, which we see as lightning. If there is some charge leftover, then extra lightning strokes will use the same “channel” and may branch out with more additional strikes, giving the lightning bolt a flickering appearance.
However, the electric field within the thunderstorm overpowers the electric field between the cloud base and the ground, which results in 70% to 80% of all lightning strikes to be contained within the cloud, rather than cloud-to-ground.
Despite the lightning strike itself having charges coming from both the ground and the sky, 99% of the flash we see is from the ground, which means that while it is known as cloud-to-ground lightning, it is ACTUALLY ground-to-cloud. In my honest opinion, they would have been better off naming it lightning and not the other, but what do I know? A lot actually 😉
At other times, a rare form of lighting may occur; it originates from the “anvil” or top of the cumulonimbus cloud. This is known as positive lightning. These occurrences are in fact so rare, that only 5% of all lightning strikes are positive. The same thing occurs as in regular (negative) lightning, but instead, the charges that descend from the cloud are positive, while the charges at the ground are negative.
Despite being a rare form of lightning, it can be very dangerous. For one, the amount of air it burns through to reach the ground is much greater than your typical lightning strike. This means that the electric fields associated with the positive strikes are much stronger than the electric field associated with the negative ones. For two, the flash duration is much longer, which means that the peak charge can be up to ten times higher than negative strikes. Some of the strikes do occur within the cloud, and all is happy, but there are times when positive lightning strikes the ground, it may strike up to 25 miles (40.2 km) away from the storm itself. For three, along with the second reason, these positive strikes have been responsible for numerous forest fires and power outages.
It is also worth noting that positive lightning is most common during the winter, and that it also more likely to happen in the dissipating stage of a thunderstorm. Positive lightning is also usually just one stroke whereas negative lightning typically branches off into more than one.
The rarest form of lightning is bipolar lightning, in which the positive charge becomes negative and the negative charge becomes positive. This form is extremely rare, although it is not dangerous. With that said, bipolar lightning is not yet fully understood.
You can stay safe by following these guidelines from the National Weather Service.
OTHER FORMS OF LIGHTNING
- Red lightning flashes that appear directly above a thunderstorm as a very large, but weak flash.
- Generally occur at the same time as powerful cloud-to-ground lightning strikes.
- May extend an upward of 60 miles (95.6 km) from the cloud top.
- Are not usually seen by the human eye, and even if they are, it is always at night. Highly sensitive cameras are sometimes able to capture them.
- Like res sprites, they emerge from the cloud top of a thunderstorm, but are NOT associated directly with cloud-to-ground lightning.
- They extent up in narrow “cones,” which “fan out” and disappear at 25 to 35 miles (40.2 to 56.3 km).
- These lightning strikes last for a fraction of a second, but have been seen by pilots.
- They are not elves, like the ones in Christmas movies, but they are rather a rapidly expanding disk-shaped form of lightning that can be a staggering 300 miles (482.8) in diameter.
- They last for a fraction of a second above thunderstorms with cloud-to-ground lightning.
- It is theorized that elves occur when an energetic electromagnetic pulse finds its way into the atmosphere.
Thunder is an acoustic shock wave from the extreme heat of a lightning strike. All thunder happens in the same way. A lightning bolt can be as hot as 50,000°F (27,760°C) although some have been as hot as 54,000°F (30,000°C). Nevertheless, it doesn’t matter because both of those temperatures are over five times hotter than the surface of the Sun! Because the lightning bolt is so hot, the air’s temperature surrounding it will rise to nearly the same temperature as the bolt itself for a fraction of a second.
As with most substances or any gas, when heated, they expand. The faster they become heated means that the rate of expansion also increases. Because the air around a lightning bolt will get heated so fast, a massive expansion occurs, resulting in the air in front of the lightning bolt being compressed, resulting in a loud acoustic shock wave.
Temperature also plays a big role in the sound of thunder and how far away it can be heard. Sound waves move faster in warm air than they do in cool air. Usually, thunder can be heard about 10 miles (16 km) away. This is because the air temperature decreases with height. On occasion, if the air temperature increases with height, the sound waves will be refracted toward the ground as they move faster in warmer air. Refraction can amplify the sound of thunder.
The weather in the equatorial regions is usually hot and humid, but the temperatures don’t usually climb over 95°F (35°C), because the majority of the Sun’s energy is used for evaporation, rain formation, and photosynthesis. This is due to the majority of solar radiation being exerted on the equatorial region of Earth, whereas the poles receive the least. This imbalance in energy drives the atmospheric circulation patterns and ocean currents, which try to balance out the energy.
As we discussed above, rising air creates low pressure areas, which are stormy systems, which is why the tropics tend to see lots of rain and storminess. At the same time, the tropics also receive a lot of sunshine. The rain plus the Sun make great growing conditions for plants. During the nighttime, temperatures generally fall into the low or mid 70s, and do not usually fall into the 60s due to substantial cloud cover during the overnight hours. Seasons do not change the temperatures that much near the equator, which is why seasons are divided into wet and dry periods.
INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
The Inter-tropical Convergence Zone is a band or line of clouds that produce rain showers and occasional thunderstorms. These bands of clouds are near the equator and they encircle the globe. These bands may stretch for hundreds of miles, but are usually broken apart into smaller bands.
This unique setup exists because of the convergence of trade winds, which we talked about earlier. In the Northern Hemisphere, the northeast trade winds will meet the southeast trade winds of the Southern Hemisphere. When they meet, warm, moist air is forced to lift which forms clouds. These clouds form the ITCZ. Storms in the tropics are short-lived, which is why when you are at an island resort, a passing thunderstorm will not put a wrench in your works. However, despite thunderstorms near the tropics being short-lived, the rainfall they produce can be exceptional; rates can be one inch (2.54 cm) or more an hour.
The wet and dry seasons for the tropics are made possible due to the Intertropical Convergence Zone. Like the jet stream, the ITCZ also “follows the Sun,” which means that it moves north in the Northern Hemisphere summer and south in the Northern Hemisphere winter. There are two wet seasons and two dry seasons as the Sun passes over the equator in March and September. The dry seasons are made possible as the Sun is farthest away from the equator in December and July.
Once you get into the Northern Hemisphere or Southern Hemisphere, there is only one dry season and one wet season. In the Northern Hemisphere, wet season is May through July and in the Southern Hemisphere, it is November through February.
Tropical cyclones are a warm-core of low pressure without any front associated with the system. They develop over tropical and subtropical regions and they have a very organized circulation. Tropical cyclones need a lot of energy in order for the storm to form and they generate a lot of energy once developed. Many scientists (and nutheads) have theorized in making these storms less intense by “cloud seeding” or dropping water absorbing material to soak up the moisture of the storm, while others are so far fetched as to suggest using nuclear weapons to disrupt the storm’s circulation. However these lunatics don’t understand that tropical cyclones, like sunspot activity, cosmic rays, and long-term orbital and axial tilt patterns play a vital role in the global climate system. Tropical cyclones help regulate Earth’s temperature, especially in the oceans.
Cyclically warmed oceans due to high sunspot activity, fuel tropical systems, which is partially why we saw active seasons during the 1850s – 1880s, 1940s, and 1950s. When oceans are warm, and the atmosphere is cooler, this causes an imbalance in the climate system leading to a recipe for disaster. With the cool atmosphere, and warm ocean, tropical cyclone development is prone to happen, in which the storm will assist in naturally cooling the oceans. This cycle repeats on multidecadal time scales in which the oceans will warm and the tropical cyclone activity will pick up. However, colder times globally have been theorized by Florida State University to produce more tropical systems. As we discussed before, the oceans retain heat for a long time as they have a high specific heat, and as the land and atmosphere cool, the system becomes unbalanced, leading to an uptick in tropical cyclone activity.
This is why we need to leave tropical cyclones alone and let the natural climate system do what it does without using nuclear weapons of all crazy ideas. If we were even able to decrease the intensity of hurricanes by manipulating the weather somehow (hasn’t happened yet nor is it likely to become possible), the absence of activity could cause the cyclically warmed oceans to form a massive storm, possibly worse than Katrina or the 1938 Rhode Island Hurricane, and that would lead to Al Gore’s hysteria across the media.
In order for a tropical cyclone to form, water temperatures need to be at least 80°F (27°C) with a water depth in the ocean of at least 150 feet (46 m). Water temperatures have to be 82°F (28°C) in order for the tropical cyclone to be sustained. There are also many other factors that come into play for tropical cyclone development. For one, a tropical cyclone has to develop within at least 300 miles (480 km) from the equator and the atmosphere needs to cool fast enough with height so that it is unstable to moist convection. There needs to be moist air near the mid-troposphere, which is at 16,000 feet (4,900 m). A vertical wind shear with values less than 23 miles per hour (37 km/h) between the surface and the upper atmosphere is also necessary.
Since tropical cyclones require sea surface temperatures of at least 80°F (27°C) to develop, it is common sense that they form near the equator. However, to the contrary, it is rare to see a tropical cyclone to develop within 5° latitude of the equator. The reason that hurricanes develop in the equatorial regions and NOT right at the equator is because of the lack of strong Coriolis force, which is what causes these systems to spin.
TROPICAL CYCLONE SIZE
Most tropical cyclones that are hurricanes are approximately 300 miles (483 km) wide. However, the diameter of these storms vary greatly all the time. Contrary to what many believe, size is NOT an indicator of hurricane strength or intensity. Take for example, Hurricane Andrew of 1992, which struck as the last Category 5 storm in the U.S.; compared to Harvey or Katrina, Andrew was a slightly smaller than average hurricane.
Size wise, Typhoon Tip of 1979 was the largest tropical cyclone on record, with a diameter of about 675 miles (1,087 km). The smallest tropical cyclone ever recorded was Tracy, which was only 30 miles (48 km) wide, when it struck Darwin, Australia on Christmas Eve in December of 1984. To have a cyclone that small is extremely rare considering the eye of most hurricanes are about 30 miles wide.
However, regardless of a hurricane’s or tropical cyclone’s size the destructive winds and intense rainfall can cover a large area. The damaging hurricane-force winds may extend outward from the eye an upward of more than 150 miles (242 km). Tropical storm-force winds can extend an outward of 300 miles (483 km) from the eye if the hurricane is large enough.
TROPICAL CYCLONE FORMATION
Warm water is what powers a tropical cyclone. As water evaporates, it rises and cools with altitude. This cooling causes the water vapor to condense into water droplets we see as clouds. Heat is released in the process of condensation and the heat released increases the temperature in that area of the atmosphere making the air lighter which allows it to rise even further. As this occurs, the denser and cooler air sinks and undercuts the warm air, which is felt as very strong wind at the surface. This is why the storm loses power and intensity as it moves over land; there is a lack of moisture and heat being supplied to the storm, causing it to dissipate quickly.
However, there is more than meets the “eye” to hurricanes. Warm water alone isn’t sufficient enough for hurricane development. One of the most important factors in hurricane formation is that there has to be an imbalance or disturbance in the atmosphere. There are four main ways that cause a disturbance.
The most common are easterly waves, which are also referred to as tropical waves. These are an inverted trough of low pressure that move westward off the western coast of Africa. This is how most tropical systems form.
Another way for tropical cyclones to develop are lines of convection, similar to a squall line, called a West African Disturbance Line (WADL). These move over western Africa and move into the Atlantic basin, and typically move faster than easterly waves.
A Tropical Upper Tropospheric Trough (TUTT) is a cold core low in the upper atmosphere that creates convection. It is not that common, but they have been known to form tropical cyclones.
The final way a tropical cyclone can form is with an Old Frontal Boundary, which are the remnants of a polar front. These can become lines of convection and have been known to spawn tropical cyclones. Atlantic storms will tend to have this be the case during early or late hurricane season in the Gulf of Mexico and/or Caribbean Sea.
Once one of the four disturbances forms, and convection becomes sustained, the storm is able to organize if atmospheric conditions are right. The system can become a depression if upper level winds are weak and water temperatures stay at 82°F (28°C).
THE PARTS OF A TROPICAL CYCLONE
Tropical cyclones are set up very uniquely; the main parts of these systems are the rainbands, the eye, and the eyewall.
Air will circulate into the storm moving toward the center in a counter-clockwise fashion (in the Northern Hemisphere). In the Southern Hemisphere, the air circulates clockwise. The air will eventually spiral out of the storm in the opposing direction of which is spun into the storm.
THE PARTS OF A TROPICAL CYCLONE: THE EYE
Winds inside the eye of the storm are 15 miles per hour (24 km/h) or less. The air inside the eye sinks lightly, creating calm conditions within the eye. The eye of a hurricane or tropical storm is generally 20 to 40 miles (32 to 64 km) in diameter. In order for the eye to form and be sustained, hurricane wind speed needs to be at least 74 miles per hour (119 km/h).
It is currently unknown how the eye forms in a tropical cyclone, but it has been theorized that it is due to sinking air within the center of the storm as well as the conservation of angular momentum, which is the speed increase in the storm as air circulates in toward the center of the storm. What is known however, is that the center of a cyclone is calm and clear of clouds. However, some physics comes into play, as Newton’s third law comes into play again. For every action, there is an equal and opposite reaction. As the storm speed increases, there is a force directed outward known as the centrifugal force. The reason that this occurs is because of the momentum of the wind within the system trying to blow straight. However, the wind does not go straight, as it moves in a circular spiral about the tropical cyclone’s center, which causes an outward pull or force. The sharper the curvature of the wind, the stronger the outward force will be.
If wind speeds are at 74 miles per hour (119 km/h) or more, the strong rotation of the storm balances inflow of air toward the center, causing air to rise 10 to 20 miles (16 to 32 km) from the center, which forms the eyewall. The rotation of the cyclone also forms a vacuum of air at the center or eye of the storm, which allows the air to flow out of the top, in which some of it gets lightly pushed back into the center of the storm. This replaces the loss of air near at the storm’s center and stops cloud development, which allows the sky to clear in the center of the storm, forming the eye.
THE PARTS OF A TROPICAL CYCLONE: THE EYEWALL
The eyewall of a tropical cyclone is a big ring of tall vertical thunderstorms surrounding the eye that produce intense rainfall and also have the strongest winds in the storm. When the structure of the eye and eyewall change, wind speed may change. The eye of the storm may shrink or grow and the eyewall may duplicate, forming two eyewalls; the inner and the outer.
Very strong cyclones sometimes have some of the rainbands organize into one ring of thunderstorms called an outer eyewall, which may take moisture from the eyewall, which causes the storm to weaken.
However, in other cases, the outer eyewall may simply replace the inner eyewall, which causes the storm to regain its intensity or become stronger than previously. This is referred to as the “eyewall replacement cycle.”
THE PARTS OF A TROPICAL CYCLONE: RAINBANDS
Rainbands are bands of circular lines of clouds and thunderstorms that circle the eyewall. These rainbands may produce short, intense downpours of rain over a large area and they may even spawn tornadoes and gusty wind, similar to what we saw with Hurricane Florence. These rainbands sometimes have gaps where there is no rain or wind.
TROPICAL CYCLONE CLASSIFICATION
Tropical cyclones are categorized into different types of storms based on wind speed.
- Tropical Depression| Cyclones with a well defined spin with sustained winds 38 miles per hour (61 km/h) or less.
- Tropical Storm | Cyclones with sustained winds between 39 miles per hour (63 km/h) and 73 miles per hour (117 km/h). These are when the storms get their names.
- Hurricane (Atlantic) | When sustained winds are at least 74 miles an hour (119 km/h).
- Typhoon (Northwestern Pacific) | See hurricane. Typhoons become “super typhoons” if winds are at least 150 miles per hour (241 km/h).
- Severe Tropical Cyclone (Southwest Pacific) | See hurricane.
- Severe Cyclonic Storm (North Indian Ocean) | See hurricane.
- Tropical Cyclone (Southwest Indian Ocean) | See hurricane.
Hurricanes are classified by wind speed into different categories, going 1 – 5. Meteorologists use what is called the Saffir-Simpson Hurricane Wind Scale.
TROPICAL CYCLONE NAMES
People have been naming hurricanes for centuries. During the 1800s, locals in the West Indies named them after saints if they fell on particular holidays, and meteorologists have named them for a long time too. During World War II, Navy and Air Force meteorologists gave the storms feminine names.
By 1953, the U.S. decided to name storms by a phonetic alphabet, starting with A and ending in W, excluding Q, U, X, Y, and Z, because of the lack of suitable names. Feminine names were used until 1978, when male names were tossed into the basket for the Eastern North Pacific. Starting in the 1979 hurricane season, masculine names were added to the Atlantic and Gulf.
This sytem works much easier, because it ends the confusion of confusing storms with other storms hundreds of miles away.
TROPICAL CYCLONE HAZARDS
While tropical cyclones produce damaging winds and intense rainfall, they can also produce several other hazards, such as storm surge, flooding, and tornadoes.
The first one of these other hazards are storm surges, which is when ocean water gets shoved toward the shore by the damaging winds associated with the storm. With the daily tides in place, a storm surge can cause higher than normal tides, called the hurricane storm tide of 15 feet (4.5 m) or more.
This can cause widespread coastal flooding, especially if this occurs at the same time as the normal high tide. Most coastal cities in the U.S. are about 10 feet above sea level, which means that the damage potential from a storm surge would be staggering. However, the slope of the continental shelf beneath the water also plays a role in how high the storm surge gets. The steeper the continental shelf, the harder it is for a storm surge to affect a community, the shallower, the higher the storm surge will be. Despite causing damage to buildings and structures on the surface, outboard vehicles can also become damaged due to a storm surge if they get slammed up against their dock, get capsized, or carried inland with the surge.
TROPICAL CYCLONE HAZARDS: FLOODING
Tropical cyclones also tend to produce inland flooding if they make landfall. Because they produce intense rainfall, they produce widespread flooding. Once the flooding starts, it may be several days or weeks before it settles down to normal.
According to the statistics, 60% of the 600 deaths due to floods in the United States were due to tropical cyclone flooding, not due thunderstorms, supercells, or derechos. 23% of the deaths were due to the tropical cyclone, was due to people drowning or abandoning their cars. And 78% of the children that died from tropical cyclones were drowned in freshwater flooding. Generally, these deaths are because people underestimate the power of moving water.
Slow-moving tropical cyclones tend to produce the most flooding because they are moving so slow, that a lot of rain will get dumped over a certain area. This is what happened with Hurricane Harvey in 2017, and Florence in 2018.
TROPICAL CYCLONE HAZARDS: TORNADOES
Tropical cyclones have been known to spawn tornadoes, which becomes a total disaster as they add to the damage potential. Typically, when spawned during these storms, they occur in the right, front quadrant of the hurricane relative to its movement, or in the rainbands. We have a hint that a tornado may have struck a location if there is uneven damage, whereas general hurricane damage is relatively evenly spaced out and nothing is left standing. Statistics show that almost every tropical cyclone, if it makes landfall, produces at least one tornado. Some produce many, while others spawn zero.
Tornadoes associated with tropical cyclones are generally weaker than ones spawned from derechos, supercells, and severe thunderstorms. Tornadoes can develop days after a hurricane dissipates, if its remnants are hanging around in an organized low pressure circulation. Historically speaking, tropical cyclone spawned tornadoes may occur at day or at night, but generally after 12 hours has gone by after landfall, tornadoes will only occur during the day.
You can stay safe during a hurricane or tropical cyclone by following these guidelines.
EL NIÑO SOUTHERN OSCILLATION (ENSO)
The weather and climate are variable, as it changes on monthly, decadal, multi-decadal, multi-century, or longer time scales due to different reasons. These changes may cause localized areas or a large area to see abnormally cold or warm temperatures for a long or short period of time, and a change in weather patterns, such as rainfall, snowfall, drought, and thunderstorm development. One of the big drivers of those changes, especially in the short-term, is ENSO.
Generally, long-term changes are due to atmospheric and ocean circulation changes, which can change due to orbital patterns, solar activity, or gravitational pulls. However, ENSO also plays a minor role in these long-term effects, but we will save that for a later topic.
Anyways, the El Niño Southern Oscillation (ENSO) is one of the recurring natural phenomena which is a part of a large global atmospheric variation. The “Southern Oscillation” refers to changes in sea level air pressure patterns in the South Pacific, between Tahiti and Darwin, Australia. When an El Niño occurs, the average air pressure is higher in Darwin, than in Tahiti. Thus, the change in air pressures in the Southern Pacific Ocean and water temperature in the Eastern Pacific are related.
El Niño simply refers to the water temperature in the Equatorial Pacific Ocean. When the conditions are warmer than average, we call this El Niño, and when we are in the cool phase, it is referred to as La Niña. The changes in water temperature range about 6°F (3°C), but despite these small differences in temperature, the weather impacts it has can be large.
We typically monitor ENSO by looking at the sea surface temperatures in the Equatorial Pacific, but we also use the Southern Oscillation Index (SOI), which is formulated by calculating the difference between the air pressure in Tahiti and Darwin. With the SOI Index having been used for well over 100 years now, we can easily reconstruct ENSO history for the past century or more.
As for measuring the Equatorial Pacific sea surface temperatures, we monitor the current conditions in four regions along the equator, which were “created” in the 1980s. The first is Niño 1, which is located between 80-90°W and 5-10°S, the second region is Niño 2, located between 80-90°W and 0-5°S, the third being Niño 3, located between 90-150°W and 5°N-5°S, and the last, Niño 4, located between 150-160°E and 5°N-5°S. Since the regions were created, they have been modified a little; Niño 1 and Niño 2 are now combined, which is referred to as Niño 1+2. In addition, a new region called Niño 3.4 was created, which lies between 120-150°W and 5°N-5°S, because it correlates inversely better with the SOI Index.
The two graphs on the right show the inverse correlation between the SOI Index and ENSO. The top shows the ocean temperature anomaly in the Nino 3.4 region starting in 1970 and ending in 2000, while the bottom shows the SOI Index for the same time frame.
When the air pressure in Tahiti is lower than than the air pressure in Darwin, the temperature in Niño 3.4 is warmer than normal, which means that ENSO is in its warm phase. When the air pressure is higher in Tahiti than in Darwin, that means that ENSO is in its warm phase.
EFFECTS OF ENSO: NORMAL / NEUTRAL CONDITIONS
Sea surface temperatures in the West Pacific are generally 14°F (8°C) higher than the waters off the coast of South America. This is because trade winds blow from east to west along the equator, which allows the cold water to upwell from deep within the ocean off the northwest coast of South America. This is because the air pressure in Tahiti is slightly higher than the air pressure in Darwin. The trade winds also push the water westward, which makes it pile higher in the Western Pacific by about 1 ½ feet (46 cm) higher. This westward movement of the water allows the Western Pacific to have a 450 foot (150 m) deep warm layer that forces the thermocline down, while it rises in the east, similar to how parcel movement works in the atmosphere. The thermocline in the east is 90 feet (30 m) deep, which is relatively shallow and much richer in nutrients than the surface layer.
EFFECTS OF ENSO: EL NIÑO CONDITIONS
When the air pressure patterns in the Southern Pacific Ocean change, so that the air pressure in Darwin is higher than in Tahiti, the trade winds will decrease in strength, and may even change direction. This allows the westward flow of water from South America to decrease, which allows the ocean water to pile up off the South American coast. This reduces the amount of upwelling occurring, which in turn pushes the thermocline deeper.
Because the thermocline is deeper and a slower westward movement of water, the sea surface temperature in the Eastern Pacific, particularly in the Eastern Equatorial Pacific, increase to the point where they are warmer than average. This warm phase is referred to as El Niño.
With El Niño in place, this changes the rain patterns from the normal West Pacific to the Central Pacific, which means that the West Pacific becomes dry.
EFFECTS OF ENSO: LA NIÑA CONDITIONS
When the air pressure in Tahiti is higher than the air pressure in Darwin, this means that the trade winds have the tendency to blow stronger, east to west, which allows a large upwelling of water in the East Pacific, as the West Pacific sees a piling up of warm water, pushing the thermocline down. This causes the Eastern Pacific to see cooler than normal sea surface temperatures, while the West Pacific sees warmer than normal temperatures. This cool phase is referred to as La Niña.
With La Niña in place, this changes the rain patterns from the normal West Pacific and pushes it farther west.
EFFECTS OF ENSO: WEATHER IMPACTS – THE JET STREAM
When the warm water’s position along the equator move back and forth, the position of where the most evaporation takes place shifts with it. This has a large effect on where the jet stream goes and where storms will track.
The normal position of the dip in the jet stream is over the Central Pacific. During an El Niño, the jet stream tends to dip in the Eastern Pacific. The stronger the El Niño is, the farther east the dip is in the jet stream. During a La Niña, the dip in the jet stream takes place further west. The dip in the jet stream, and its position, can have large effects on the weather we see in North America for a given period of time.
In El Niño phases, the eastern track of the dip in the jet stream usually sends the storm track with lots of tropical moisture into California south of where it usually is, in the Pacific Northwest. The stronger the El Niño, the farther south the tropical moisture will be shifted.
EFFECTS OF ENSO: WEATHER IMPACTS – TROPICAL CYCLONES
If we look back at the statistics and data, we will notice that tropical cyclone activity in the North Atlantic Ocean tend to see the biggest effects from El Niño and La Niña than any of the other basins of water.
Statistics show, that during moderate and strong El Niños, the North Atlantic tends to experience a large reduction in tropical cyclones, a 60% reduction in hurricane days, and an overall reduction in the intensity of these storms. It is theorized that the reason for this is because of the stronger than normal westerly winds that form between the North Atlantic and Caribbean during El Niño events.
EFFECTS OF ENSO: WEATHER IMPACTS AROUND THE GLOBE
Below are some maps which show the areas where El Niño and La Niña have big effects. The areas that aren’t highlighted don’t typically see a significant change from normal during these events.
Many meteorologists, especially television and government meteorologists, in my opinion, put too much focus on ENSO impacting seasonal weather; they rely too heavily on it for winter and summer forecasts. In my expert opinion, ENSO does not have large effects on temperature anywhere, other than the Pacific. However, I think ENSO plays a bigger role in the amount of rain or snow any given place receives, much more than the maps above say. I could go into a full discussion on this, but I will save that for a future blog post.
Winter weather is perhaps one of the most interesting weather that one gets to experience. It is certainly The ClimateGuy‘s favorite!
Anyways, a winter storm is a system that produces frozen precipitation; snow, sleet, or freezing rain. Despite snow being pretty, it can be very dangerous. Although winter weather is not a direct threat to people itself, most deaths occur due to traffic accidents and hypothermia, but people can also die from heart attacks while shoveling snow, especially if they are over age 40.
WINTER STORM FORMATION
As with your typical storm system during the warmer months, winter storms form due to energy. Atmospheric conditions also have to be right in order for the storm to be a winter storm.
Despite the having conditions right in order for the system to develop and organize, the main thing you need is cold air in place. Temperatures must be below freezing in the clouds, the air, and near the Earth’s surface. Lifting is also necessary for winter storm development.
Lifting is needed to force the warm, moist air to rise and condense into clouds. With clouds, you get precipitation, and with winter storms, that precipitation is frozen when it reaches the ground. Lifting may occur along a frontal boundary, where the warm air collides with the cool air, which forces the warm air to rise above the cold air. Lifting may also be done orographically, which means the air flows up a mountainside.
The last main thing you need in moisture in place. If you have moisture, clouds are able to form, which means that precipitation will eventually occur. If air gets blown across a land mass by being blown across large body of water, such as one of the Great Lakes or the oceans, combined with lifting, this allows clouds to form, which will eventually organize into a winter storm.
WINTER STORM TYPES
- Blizzards | Blizzards are very dangerous (and fun) types of winter storms. They have a combination of wind, snow (lots of it), and frigid temperatures. Strong, gusty winds can blow the snow into drifts, causing more problems than normal snowfall will. While the snow is blowing, visibility can be reduced to near zero.
- Lake Effect Snowstorms | Lake effect snow does not form due to low pressure systems, contrary to your typical snowstorm. These snow events generally occur in areas that surround or are near the Great Lakes, more specifically south and east of the Lakes. The formation of these storms are due to cold, dry air flowing across the lakes. The flow of the air picks up moisture from the Great Lakes, which eventually results in cloud formation, in which precipitation will fall to the ground as snow.
- Ice Storm | Ice storms are another type of winter storm which are deadly. Ice accumulation is generally more than 0.25 inch (0.635 cm) on surfaces, such as roads, sidewalks, cars, and trees. Despite being such a small accumulation of ice, the weight of it can tear down tree branches and power lines.
- Snow Squalls | Snow squalls are a short, but intense snow showers that are accompanied with gusty winds. Accumulation is typically small, but have been known to produce considerable snowfall in a short time period. Snow squalls typically occur in the Great Lakes region, but have been known to occur in the Mid-Atlantic and Northeastern states.
Snow is the most
common fun form of precipitation associated with winter storms. Typically, the precipitation associated with these storms start out as snow in the top layer of the clouds. Snowflakes are simple collections of ice crystals that bond together as they fall to the Earth’s surface. Snowfall will stay as snow as it falls and reaches the ground if temperatures are below freezing. However, in some cases, temperatures can hover just above freezing and snow can still accumulate considerably, if surfaces are cold enough. Snow can fall to the ground with temperatures in the mid to upper 40s as well, but the freezing level of the atmosphere has to be really low in that particular location.
If snow falls very lightly, with very few flakes, the occurrence is called snow flurries. These flurries typically fall in a small period of time, generally less than an hour. No accumulation generally occurs from snow flurries, but if they last long enough and/or if the surface is really cold, a slight dusting is possible.
If snow falls lightly with varying intensity from minute to minute or hour to hour, we call this snow showers. Light accumulation is possible.
Snow squalls are similar to a snow shower, but they generally last much shorter, but they have been known to produce a lot of snow in short periods of time. Snow squalls are usually associated with gusty wind.
Snowstorms or blizzards are large, organized winter storms that bring exceptional snowfall and high, gusty winds sustained at 35 miles per hour (56.3 km/h) or more. Due to the wind, snow gets blown around, which reduces visibility considerably, down to 1/4 mile (0.4 km), and forms snow drifts.
Sleet is another form of wintry precipitation, which forms when snowflakes are partially melted as they fall toward the ground. In a deep layer of the atmosphere, sleet falls in the form of snow, but will partially melt in a shallow layer of warmer air as it falls. As it reaches the ground, it will become frozen rain drops with contact in the freezing air. Sleet bounces upon impact with the surface.
FREEZING RAIN / ICE
Freezing rain sucks, let’s just get it honest. Freezing rain (ice) occurs when snowflakes fall from clouds in a shallow cold layer in the atmosphere, in which they fall to the ground as rain, but refreeze upon contact with the Earth’s surface or anything that is at the freezing point or below. This creates a layer or “glaze” of ice on the ground, trees, power lines, or other objects. If accumulation is over 0.25 inch (0.635 cm), then the event is referred to as an ice storm, which have been known to knock out power.
THE UPPER AIR
The upper air is the Earth’s atmosphere above 5,000 feet (1,500 m). Almost all weather we have at the ground level comes from the upper air. When meteorologists are forecasting the weather, looking at the upper air is the first thing they do before they look at what is happening at the surface. The image below shows the state of the atmosphere for a specific time one day at 18,000 feet (500 millibars) above the Earth’s surface. The blue lines on the map show places of low and high pressure regions in the atmosphere.
Most of us think that warm air rises because warm air is less dense than cooler air. When the temperature rises, the density falls, and vise versa. But there are also other reasons for why warm air rises and cooler air sinks. Air of lower density doesn’t rise automatically because there is no force being exerted on the warm air to force it upward. This takes us back to Newton’s first law of physics: an object at rest will stay at rest, unless another force is exerted on the object.
It has been theorized that gravity is the force that pulls denser and cooler air toward the Earth’s surface. This cooler air will spread and go under the warm air, which forces the warm air up. This is one of the reasons for why storms will develop along weather fronts. These weather fronts mark the boundaries between cooler and warm air masses and where they meet. The cooler air sinks and goes under the warm air, forcing it to rise. When the warm air rises, thunderstorm are likely to develop as the air mass cools down.
These pockets of air are known as parcels in atmospheric science. There are many things that we assume when it comes to the parcel theory. These assumptions are not always accurate, but they are ideal.
The first assumption is that when you are in a stable atmosphere, the parcel becomes cooler than the surrounding air as it rises, whereas in an unstable atmosphere, the opposite happens; the parcel stays warmer than the air around it. When the air mass is cooler than the air surrounding it, the parcel will rise slower, and then come to a stop altogether. When the air mass is warmer than the surrounding air, then the parcel will continue to rise.
We also assume that the cooling rate of the parcel remains constant in both a stable and an unstable atmosphere. Stability and instability are based on the vertical temperature profile of the atmosphere. We assume that the ratio of moist to dry air stays constant as the parcel rises or sinks. Another assumption is that there is no other heat source being applied to the parcel. A parcel that has a relative humidity less than 100% (unsaturated) will become 100% saturated by cooling 5.5°F every 1,000 feet (9.8°C every 1,000 m). When a parcel is already saturated, it will cool at a lower rate, because condensation releases heat. When you have outside influences applying extra heat to a parcel, the cooling rate becomes much slower.
Parcels help us figure out the stability of the atmosphere. When the temperature of the parcel decreases to where it is cooler than the air surrounding it, the parcel will get denser, and with the force of gravity, the parcel will stop rising, or even start to sink. This is known as negative energy, which means that the atmosphere is stable at that level. When the temperature of the parcel is higher than the air surrounding it, the parcel will become less dense, which will allow it to rise. This is known as positive energy, which means that the atmosphere is unstable at that level.
ATMOSPHERIC STABILITY & INSTABILITY
Stability and instability of the atmosphere have a lot to do with cloud development. In fact, it is one of the most important factors in cloud formation. Stability is simply the tendency for a parcel of air to rise.
Air cools as it rises, which allows it to condense into clouds. Air becomes warmer and drier as it sinks. A parcel will slowly exchange properties with other air masses, and even do it slower with a larger air mass. When no heat is exchanged, we call that an adiabatic process. As we learned before, air masses (parcels) cool 5.5°F every 1,000 feet (9.8°C every 1,000 m); this is called the dry adiabatic rate. As the air rises, it cools, and its relative humidity increases to which the dew point will equal the temperature at some point, which allows the air to become saturated. With further lifting of the air, this allows clouds to condense and form with the assistance of latent heat being applied to the parcel. Condensation is a warming process because the water molecules must release energy in order for their movement to slow down. The heat being released from the water molecules as the cloud forms slows the cooling rate of the saturated parcel, which is referred to as saturated air. The cooling rate of the saturated air is called the moist adiabatic rate.
The moist adiabatic rate is generally less than the dry adiabatic rate, but it can vary, due to the variations in temperature and moisture content in the atmosphere. For the moist adiabatic rate, we generally use the cooling rate of 3.3°F every 1,000 feet (6°C every 1,000 m).
In order to find a parcel’s stability, you have to compare the parcel’s temperature to the temperature of the surrounding air mass. If the temperature of the parcel is less than the temperature of the surrounding air, then it is denser than the surrounding air, which allows the parcel to sink. This is referred to as stable air. If the opposite occurs, and the parcel’s temperature is warmer than the surrounding air, the parcel is less dense, which allows it to rise. This is referred to as unstable air.
In stable air, the environmental lapse rate is 2°F for every 1,000 feet (4°C every 1,000 m).
- Absolute Stability | When the environmental lapse rate is less than the moist adiabatic rate, the parcel will cool quicker than the air encircling the parcel. This means that the parcel will not rise. If a force is exerted on the parcel, such as during orographic uplift or uplifting along a front, then the parcel will spread out into a thin, stretched-out cloud like cirrostratus, altostratus, nimbostratus, or stratus, and return to its original starting elevation. A low to moderate adiabatic rate enhances the atmosphere’s stability. Warm air aloft and cool air below result in a low to moderate environmental lapse rate. Fog and haze, like strato-form clouds, also form in a stable atmosphere because of the sinking air above. This may result from a high pressure area, which prevents the air underneath from lifting. Because the air cannot lift, means that particles of water, air, and dust are closer together as condensation nuclei.
- Absolute Instability | When the dry adiabatic rate is less than the environmental lapse rate. The parcel becomes warmer and less dense than the air encircling the parcel. The parcel will rise due to the buoyant forces.
- Neutral Stability | When the environmental lapse rate is equal to the dry adiabatic rate. The parcel will move and stop, then remain in that position.
- Conditional Stability | Occurs when the environmental lapse rate is between the moist and dry adiabatic rates. A parcel will sit in a dip or depression, and with a light force exerted on the parcel, it will move up diagonally, then oscillate back and forth until it goes back to its original position.
- Conditional Instability | The most common state of the atmosphere. It is similar to conditional stability, but there is a greater force applied to the parcel causing it to continuously move without oscillating back and forth. It also depends upon whether the rising parcel of air is saturated or not.
- Convective Instability | When air is forced to lift, it becomes more unstable as the top layer cools faster than the bottom layer. This steepens the environmental lapse rate. If the bottom layer is moist and the top layer is dry, then this may steepen the environmental lapse rate even further. This means that less lifting is needed to steepen the environmental lapse rate. This atmospheric condition is often associated with severe thunderstorms.
HEAT INDEX & WIND CHILL
Some places around the world are characterized as being “hot.” However, heat isn’t just always the threat when it comes to high temperatures. Humidity is usually associated with hot weather, especially in tropical regions. In deserts, humidity is low because the air is dry, therefore, the heat is the threat.
Heatwaves are tend to kill the most people in the United States than all tornadoes, floods, and hurricanes combined. An estimated 124 people die each year due to heatwaves in the U.S.
Heatwaves happen every summer in both hemispheres and they occur due to the buildup of heat near the surface. Heatwaves are caused by high atmospheric pressure 10,000 to 25,000 feet (3,000 to 7,600 m) above the Earth’s surface. This ridge of high pressure resides in the middle layers of the troposphere on the equator side of the jet stream. The high pressure will strengthen over several days or weeks, over a large area. It strengthens because summertime weather patterns do not change as quickly as they do in the wintertime. Because the weather patterns change slowly in the summer means that the high pressure area will also move slowly. During a heatwave, the high pressure area will sink toward the Earth’s surface, which makes the sinking air act like a dome, which traps heat inside. This is where we get the term “heat dome” from. The trapped heat will not lift. Without the air lifting, there will be very little convection taking place, which will not allow water to evaporate to form cumulus or cumulonimbus clouds. Without these clouds, rain chances are slim to none.
Humidity (relative humidity) is the measure of the amount of water vapor in the air, divided by the maximum amount the air can hold. This is expressed as a percent. Therefore, a relative humidity of 80% means that the air contains 4/5 of the water vapor the air can hold. However, the total amount of water vapor the air can hold depends upon the temperature. The higher the temperature, the more water vapor the atmosphere can hold. The cooler the temperature, the less water vapor the atmosphere can hold. When the humidity is high, the temperature will feel around 15°F (23°C) warmer and this can be a big health risk if you are outside.
- At 32°F (0°C), the air can hold 0.16 ounce of water.
- At 80°F (27°C), the air can hold 1 ounce of water.
Like there are hot places, there are also cold places, such as the Arctic and Antarctic. In these regions the humidity is no problem, but the wind chill is. In these cold regions, or in any given area where the temperatures may get cold for a certain amount of time, the wind chill is a big health risk, just as much as the humidity can be. Wind chill is what we feel as the temperature based on the rate of heat loss from exposed skin from the flow of the wind and the extremely cold temperatures.
When the wind speed goes up, the body loses heat, which makes the skin temperature fall. For example, if the temperature is 10°F (-12°C), with a wind speed of 25 miles per hour (13 kts or 24 k/ph), the wind chill would be -11°F (-24°C). Besides the health risks due to the wind chill making the temperature feel colder than it actually is against your skin, the wind chill also increases risk for frostbite or hypothermia.
In meteorology, we use “real feel” temperatures, no matter the temperature; whether freezing, cold, chilly, cool, mild, warm, hot, or extremely hot.
When there are hot temperatures, we use what is called the heat index chart.
When the temperatures are really cold, we use the wind chill chart.
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US Department of Commerce, and NOAA. “Tropical Cyclone Structure.” NWS JetStream, NOAA’s National Weather Service, 11 Oct. 2017, http://www.weather.gov/jetstream/tc_structure.
US Department of Commerce, and NOAA. “Types of Derechos.” NWS JetStream, NOAA’s National Weather Service, 2 Oct. 2017, http://www.weather.gov/jetstream/derecho_types.
US Department of Commerce, and NOAA. “Types of Precipitation.” NWS JetStream, NOAA’s National Weather Service, 28 Sept. 2017, http://www.weather.gov/jetstream/preciptypes.
US Department of Commerce, and NOAA. “Types of Thunderstorms.” NWS JetStream, NOAA’s National Weather Service, 10 Oct. 2017, http://www.weather.gov/jetstream/tstrmtypes.
US Department of Commerce, and NOAA. “Weather Impacts of ENSO.” NWS JetStream, NOAA’s National Weather Service, 2 Oct. 2017, http://www.weather.gov/jetstream/enso_impacts.
US Department of Commerce, and NOAA. “Where and When Do Derechos Occur.” NWS JetStream, NOAA’s National Weather Service, 2 Oct. 2017, http://www.weather.gov/jetstream/derecho_climo.
US Department of Commerce, and NOAA. “Wind Chill.” NWS JetStream, NOAA’s National Weather Service, 6 Oct. 2017, http://www.weather.gov/jetstream/chill.
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