Lower wind Lower wind Cloud street Cloud street U p e r w i n d U p e r w i n d Figure 9-14.  Cloud street wave. 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] 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] 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. 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. 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  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  9-11