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Thunderstorms


Thunderstorms are cumulonimbus clouds that produce thunder and lightning. The figure below shows the average number of days that thunderstorms occur over the United States. The greatest occurrence of thunderstorms occur in the southeastern United States, with a secondary maximum over the Colorado Rockies. These regions frequently have all the necessary conditions for thunderstorm formation.


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In order for a thunderstorm to form, three "ingredients" must be present:

1. Moisture
2. Instability
3. A Lifting Mechanism


Sources of Moisture
Moisture is very important in thunderstorm formation because it "fuels" the thunderstorm. Typical moisture sources are large bodies of water, such as the Gulf of Mexico, Atlantic Ocean or Pacific Ocean. The southeastern United States can tap into moisture from two of these sources (Gulf of Mexico and Atlantic Ocean). This is one reason why this region has the greatest frequency of thunderstorms in the United States.





Instability
Air is said to be unstable if it continues to rise after being given a slight "push" upward. Conversely, air is considered to be stable if it returns to its original position after being "pushed" upward. In order for thunderstorms to develop, air needs to be unstable. Air is most likely to be unstable when warm, moist air is present at the surface and cold, dry air is present aloft.





Lifting Mechanism
Another ingredient that must be present is a lifting mechanism to give the air the initial "push" upward. There are several ways in which air can be lifted. Lifting primarily occurs along fronts (cold, warm, stationary, or occluded fronts). Air can also be lifted as it flows over hills or mountains.



Locations where these three"ingredients" come together are most likely to experience a thunderstorm.




Stages of a Ordinary (Non-Severe) Thunderstorm

Many non-severe thunderstorms go through a life cycle consisting of three distinct stages. This life cycle generally lasts one to two hours.


Towering Cumulus Stage
The first stage is the towering cumulus stage, or growth stage. The warm, moist air rises and cools, eventually condensing into a cumulus cloud. As condensation occurs, it warms the air (remember, condensation is a warming process), keeping the air inside the cloud warmer than the air around it. This keeps the air unstable and allows the cloud to keep growing vertically. During this stage, updrafts keep the water droplets and ice crystals suspended in the cloud. There is no precipitation, and generally no lightning, or thunder during this stage. As the cloud builds to altitudes where the temperature is below freezing, large raindrops and even small hail begin to form. Eventually, the raindrops and small hail become heavy enough that the updraft cannot keep them suspended in the cloud and they begin to fall as precipitation. These falling particles, and evaporation and cooling of air near the cloud boundaries, creates a downdraft, which signifies the beginning of the next stage.





Mature Stage
The appearance of downdrafts marks the beginning of the mature stage. During this stage, updrafts and a downdrafts are present and the thunderstorm is at its most intense state. The cloud grows so high, that it reaches a stable part of the atmosphere (possibly the stratosphere) and cannot grow any higher. The top of the cloud spreads out and forms an anvil shape. Lightning, thunder, heavy rain and possibly small hail are produced during this stage. Sometime after the storm enters its mature stage, it eventually begins to dissipate. This signifies the beginning of the next stage.





Dissipating Stage
During this final stage, the updrafts weaken and the downdrafts dominate the thunderstorm. The thunderstorm usually does not last much longer after this occurs because the updrafts were providing the thunderstorm with their "fuel", the warm, moist air from the surface. Without the warm, moist air, cloud droplets stop growing and only some light precipitation remains. Many times, the lower portion of the cloud evaporates and the only thing left of the thunderstorm is the anvil.

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