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annually. Additionally, in the southeastern United States, especially Florida, between 30 and 50 thunderstorms occur in an average year. Thunderstorms in the cool seasons usually occur in conjunction with some forcing mechanism, such as a fast moving cold front or a strong upper-level trough. [Figure 9-15] 

The lifecycle of an air-mass or ordinary thunderstorm consists of three main stages: cumulus, mature, and dissipating. The term “ordinary” describes the type of thunderstorm consisting of a single Cb, since other types of thunderstorms (described below) can occur in a uniform large scale air mass. The entire lifecycle takes on the order of an hour, though remnant cloud from the dissipated Cb can last substantially longer. 

The cumulus stage is characterized by a cumulus growing to a towering cumulus (Tcu), or cumulus congestus. During this stage, most of the air within the cloud is going up. The size of the updraft increases, while the cloud base broadens to a few miles in diameter. Since the cloud has increased in size, the strong updraft in the middle of the cloud is not susceptible to entrainment of dryer air from the outside. Often, other 

smaller cumulus in the vicinity of the Tcu are suppressed by general downward motion around the cloud. Towards the end of the cumulus stage, downdrafts and precipitation begin to form within the cloud. On some days, small cumulus can be around for hours, before Tcu form, while on other days, the air is so unstable that almost as soon as any cumulus form, they become Tcu. [Figure 9-16] 

As the evolution of the thunderstorm continues, it reaches its mature stage. By this time, downdrafts reach the ground and spread out in what are known as downbursts or microbursts. These often lead to strong and sometimes damaging surface winds. Gliders should not be flown or exposed to microbursts and the associated windshears. While increased headwinds can momentarily improve performance, the distance from the landing area may be increased beyond the capabilities of the glider. Attempting to launch with possible windshears can result in fatal consequences. 

Note: Launching of a glider either behind a towplane or ground launch is risky. Given the size of windshears, the towplane could be in a tailwind situation while the glider 

