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This module compiled with information courtesy of the official NOAA Storm Spotters Guide.
 
SECTION TWO:
GLOSSARY As in the other sections, you can click on the glossary image wherever you see it, and the glossary will open in another window. Just close that window when you are ready to continue.
THE THUNDERSTORM:

We must obtain a basic understanding of the thunderstorm before we can hope to understand tornadoes, hail, and other phenomena which are produced by a thunderstorm. Sometimes it's convenient to think of a thunderstorm as a solid object floating in the sky. Actually a thunderstorm is a process which takes heat and moisture near the earth's surface and transports it to the upper levels of the atmosphere. The byproducts of this process are the clouds, precipitation, and wind.

At any given moment, there are nearly 2,000 thunderstorms in progress around the world. Most of these are beneficial, bringing need rainfall to farmlands and reservoirs. Only a small fraction (less than 1 percent) of these storms are classified as severe, producing large hail 3/4 inch in diameter or larger and/or strong downburst wind gusts to 58 mph (50 knots) or greater. An even smaller fraction of these storms produce tornadoes. Thus, although a storm is theoretically capable of producing severe weather, only a few storms will actually do so.

In the U.S., the Florida Peninsula and the Southeast Plains of Colorado have the highest thunderstorm frequency. Although the greatest severe weather threat is in the U. S. from Texas to southern Minnesota, it is important to note that no place in the U.S. is completely immune to the threat of severe weather.

ATMOSPHERIC CONDITIONS FOR THUNDERSTORM DEVELOPMENT:

All thunderstorms, severe or not, must have three conditions present in order to form. The first necessary condition is moisture in the lower and mid levels of the atmosphere. As air rises in a thunderstorm updraft, moisture condenses into small water drop which form clouds (and eventually precipitation). When the moisture condenses, heat is released into the air, making it warmer and less dense than its surroundings. The added heat allows the air in the updraft to continue rising.

The second necessary condition is instability. If the airmass is unstable, air which is pushed upward by some force will continue upward. An unstable airmass usually contains relatively warm (usually moist) air near the surface and relatively cold (usually dry) air in the mid and upper levels of the atmosphere. As the low-level air rises in an updraft, it becomes less dense than the surrounding air and continues to rise. This process is often augmented by added heat due to condensation as discussed above. The air will continue to move upward until it becomes colder and more dense than its surroundings.

The third necessary condition is a source of lift. Lift is a mechanism for starting an updraft in a moist, unstable airmass. The lifting source can take on several forms. The most common is differential heating. As the sun heats the earth's surface, portions of the surface (and the air just above it) will warm more readily that the nearby areas. These "warm pockets" are less dense than the surrounding air and will rise. If the air has sufficient moisture and is unstable, a thunderstorm may form.

The source of lift may also be mechanical in nature. Moist air flowing up the side of a mountain may reach a point where it is less dense than its environment, and thunderstorms may develop. This is common on the eastern slopes of the Rocky Mountains during the summer. Advancing cold fronts, warm fronts, outflow boundaries, drylines, and sea breeze fronts also act as triggers by lifting moist, low-level air to the point where it is warmer and less dense than its environment at which time storms can form.

GLOSSARY
THE THUNDERSTORM LIFE CYCLE:
All thunderstorms, whether or not they become severe, progress through a life cycle which may be divided into three main stages. The developing stage, called the cumulus or towering cumulus stage, is characterized by updraft.
As updraft develops, precipitation is produced in the upper portions of the storm. As the precipitation begins to fall out of the storm, a downdraft is initiated. At this time, the storm enters its mature stage. The mature stage is marked by a coexistence of the updraft and downdraft within the storm.
When the downdraft and rain-cooled air reach the ground, the rain-cooled air spreads out along the ground and forms a gust front. Usually the winds associated with the gust front are not severe, but in extreme cases, a downburst can develop and produce severe wind gusts. Eventually, a large amount of precipitation is produced and the storm becomes dominated by downdraft. At the ground, the gust front moves out a long distance from the storm and cuts off the storm's inflow. This begins the dissipating stage of the storm.
CONVECTIVE VARIABLES:

The three ingredients listed above are necessary for the development of thunderstorms. Recent research has found that if the environment (wind, moisture, instability) of a storm is changed, then the type of storm (multicell, supercell, etc..) which is favored to exist may change as well.

The amount of vertical wind shear is the storm's environment is critical in determining what type of storm will form. Vertical wind shear is defined as a change in wind direction or speed with height. If the amount of vertical shear is low (little change in wind speed or direction with altitude) then multicellular storms with short lived updrafts will be favored. Low values of vertical wind shear result in weak inflow to a storm. Because the inflow is weak, the outflow from the rainy downdraft area will push the gust front our away from the storm. This, in turn, will cut off the storm's source of warm, moist, unstable air, resulting in a storm with short lived updrafts. Precipitation which is produced will fall through the storm's updraft and contribute to the updraft being short lived. The following picture depicts a storm in a low shear environment

As vertical wind shear increases, storms with longer lived updrafts will be favored. Stronger vertical wind shear results in stronger inflow to the storm. The gust front will be held close to the storm, and the storm will have access to the source of warm, moist air for a much longer time. As a result, the storm's updraft will tend to last longer when the environment has strong vertical wind shear. Precipitation will tend to fall downwind from the updraft rather than through the updraft. This enables the updraft to continue for relatively long periods of time. The picture to the right depicts a storm which developed in a high shear environment.

Closely related to the concept of vertical wind shear is the veering of the wind with height in the lowest mile or so of the atmosphere. Veering is defined as a clockwise turning of the wind direction as we move up through the atmosphere. It is possible to make a rough check of veering while spotting. If there are two layers of clouds in the lower levels of the atmosphere, look closely at which direction those cloud layers are moving. If the direction turns clockwise between the lower and upper layers, then veering is present.

Computer simulations and observational studies have suggested that veering of the low level wind is instrumental in the production of storm rotation. If the wind speed is sufficiently strong (usually 30 mph or greater) and veering of the wind with height is present, then horizontal "rolls" may develop in the lower levels of the atmosphere. These horizontal "rolls" may then be tilted into a vertically oriented rotation by a storm's updraft. The updraft can also "stretch" the vertical rotation and increase the rate of rotation. Once this vertical rotation has been established, a mesocyclone can develop which may produce a tornado or significant other weather.

Variations in moisture and instability can also have an effect on thunderstorms. If the amount of moisture in the atmosphere is low (as might be found on the High Plains), the storms will tend to have high cloud bases. Small amounts of precipitation will fall from these storms, but they will typically have strong downdrafts. If moisture levels in the atmosphere are high (as might be found in the Southeast), then storms will have low cloud bases. Copious amounts of precipitation will reach the ground usually accompanied by weak downdrafts. A rule of thumb to keep in mind is: the higher the cloud base, the better chance for dry microbursts. The lower the cloud base, the better the chance for flash flood producing rainfall.

The amount of instability which is present plays in important role in the strength of a thunderstorm's updraft and downdraft. If the instability is low, then a storm's drafts will probably not be strong enough to produce severe weather. If the storm's environment has high instability, then the storm's drafts will be stronger, and the storm will have a better chance or producing severe weather.

Another important factor in the storms environment, although not as critical as the above mentioned factors, is the presence of a mid level capping inversion. The mid level capping inversion is a thin layer of warm air between the low level moist air and the upper level (usually dry) air. If the mid-level cap is weak or is not present, then the storms will usually form early in the day before the sun;s strong heating can produce high amounts of instability. A number of storms may form, but the storms will generally be weak and poorly organized. If the mid-level cap is strong, then storms will not form at all. The very warm mid-level temperatures will literally act as a lid, preventing updrafts from going above the cap.

A mid-level cap of moderate strength is preferred for the development of severe storms. A moderate cap will prevent weak storms from forming, thus "saving up" the atmosphere's instability. When storms do form, usually in the mid to late afternoon, only the strongest updrafts will be able to break the cap and continue to develop. These few storms can take advantage of the high instability which is present, with little competition from nearby storms, and possibly develop into severe storms.

GLOSSARY
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