Thunderstorms and Severe Weather

Table of Contents:
Information provided by the National Weather Service

   Thunderstorms and Atmospheric Conditions
   The Thunderstorm Life Cycle
   Thunderstorm Types
   Visual Aspects of Thunderstorms
   Wall Clouds and Other Lowerings
   Non-Tornadic Severe Weather Phenomena
   The Tornado
navagation bars

     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.

Atmospheric Conditions for a 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, 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.

The Thunderstorm Life Cycle


Last Modified: Mon June 29, 1998