8.6.1: Thunderstorms
Thunderstorms have awed, intrigued, and inspired humans with their awesome force and power. There are two basic kinds of thunderstorms, air mass and severe. Air mass thunderstorms are usually created by convective uplift of warm, moist, and unstable air. Have you ever been surprised by a sudden downpour of thunderous rain on what was up to that point a pretty nice day? If so, it was probably an air mass thunderstorm. Air mass thunderstorms typically do not have very high winds, hail, or much lightning associated with them. Severe thunderstorms , however, do and may even spawn tornadoes. Severe thunderstorms tend to form along strong cold fronts where the air on either side is very different, the atmosphere is very unstable, and wind shear aloft is prevalent. Regardless of type, both kinds of thunderstorms tend to go through the same basic stages of development. We'll use the air mass thunderstorm to describe the stages of development here.
Stages of Thunderstorm Development
The initial stage of development is called the cumulus stage . During this stage warm, moist, and unstable air is lifted from the surface. In the case of an air mass thunderstorm, the uplift mechanism is convection. As the air ascends, it cools and upon reaching its dew point temperature begins to condense into a cumulus cloud. Near the end of this stage precipitation forms.
The second stage is the mature stage of development. During the mature stage warm, moist updrafts continue to feed the thunderstorm while cold downdrafts begin to form. The downdrafts are a product of the entrainment of cool, dry air into the cloud by the falling rain. As rain falls through the air it drags the cool, dry air that surrounds the cloud into it. As dry air comes in contact with cloud and rain droplets they evaporate cooling the cloud. The falling rain drags this cool air to the surface as a cold downdraft.
In severe thunderstorms the region of cold downdrafts is separate from that of warm updrafts feeding the storm. As the downdraft hits the surface it pushes out ahead of the storm. Sometimes you can feel the downdraft shortly before the thunderstorm reaches your location as a cool blast of air.
The final stage is the dissipating stage when the thunderstorm dissolves away. By this point, the entrainment of cool air into the cloud helps stabilize the air. In the case of the air mass thunderstorm, the surface no longer provides enough convective uplift to continue fueling the storm. As a result, the warm updrafts have ceased and only the cool downdrafts are present. The downdrafts end as the rain ceases and soon the thunderstorm dissipates.