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

At any given moment, there are roughly 2,000 thunderstorms in progress around the world. Most of these storms are beneficial, bringing needed rainfall to farmlands and reservoirs. Only a small fraction (less than 1 percent) of these storms is classified as severe, producing large hail 3/4 inch in diameter or larger and/or strong downburst wind gusts of 58 miles an hour (50 knots) or greater. A small fraction of the severe storms produce tornadoes. Thus, although any thunderstorm is theoretically capable of producing severe weather, only a very few storms will actually produce large hail, severe downburst winds, or tornadoes.

In the United States, the Florida Peninsula and the southeast plains of Colorado have the highest thunderstorm frequency. Relatively small thunderstorms occur about once a year in Alaska and 2-3 times a year in the Pacific Northwest. Although the greatest severe weather threat in the United States extends from Texas to southern Minnesota, it is important to note that no place in the United States is completely immune to the threats of severe weather.

Atmospheric Conditions for Thunderstorm Development

All thunderstorms, whether or not they become severe, must have three conditions present in order to form. The first necessary condition is moisture in the lower to mid levels of the atmosphere. As air rises in a thunderstorm updraft, moisture condenses into small water drops 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 earth's 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 source is called differential heating. As the sun heats the earth's surface, portions of the surface (and the air just above the surface) will warm more readily than 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 can 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 the low-level air is warmer and less dense than its environment at which time thunderstorms can form.
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