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Tornado-Producing Clouds
Tornadoes are produced by one type of cloud, known as cumulonimbus. These are towering giants that can rise to over 50,000 feet. When they become severe in nature, they are called supercell storms. In addition to tornadoes, these clouds are responsible for the majority of all severe weather, including heavy rains, severe winds, dangerous lightning and damaging hail.
Thunderstorm Lifecycle
Thunderstorms begin life as small, puffy and innocent-looking cumulus clouds. These clouds are low-level clouds, forming below 6,500 feet. If the atmosphere is unstable enough, these cumulus clouds will begin to grow vertically. As they begin to billow upwards, they become towering cumulus, or cumulus congestus clouds. At this point, a strong updraft forms within the cloud. The next stage involves the development of a strong downdraft, producing a mature thunderstorm. This downdraft is responsible for producing the heavy rains and hail. This is also the stage at which the thunderstorm has the potential to form a tornado.
Tornado Formation
Tornado formation requires both a mature thunderstorm and wind shear. Wind shear occurs when there are strong winds at all levels that turn with height in a clockwise direction, producing a horizontal circulation. This circulation is then tilted vertically by the thunderstorm's powerful updraft. This creates what is known as a mesocyclone within the supercell, a 2- to 6-mile wide area of circulating updraft. It is here that a wall cloud can develop. This wall cloud represents a lowered cloud base that descends below the mesocyclone. It is the last stage prior to the formation of a tornado. If the mesocyclone circulation intensifies enough, it will reach the ground as a tornado.
Warning Signs In the Clouds
There are several signs that indicate the potential of tornado formation in a given cumulonimbus cloud. First, clouds will overshoot the top of the thunderstorm's anvil top. This is due to the enormous power of the updraft. This overshoot will have the appearance of a bubble or dome-shaped projection. If this overshoot lasts for more than 10 minutes, it is an indication that the storm is strong enough to produce severe weather, including tornadoes. Second, a wall cloud will develop. This wall cloud will be persistent, lasting 10 to 20 minutes, and exhibit strong circulation. Additionally, there will be a strong surface inflow, with winds of 25 to 35 miles per hour.
Location of Cumulonimbus Clouds
Cumulonimbus clouds typically occur along cold fronts. These fronts represent areas where cold, dry polar air is colliding with warm, moist maritime air. This interaction creates the atmospheric instability and uplift needed to produce cumulonimbus clouds and tornadoes. Supercells are most likely to occur over the Plains, an area known as tornado alley, during the spring.