Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance  may exceed 3,000 feet per minute, so it is inadvisable to operate a small UA in an area of rapidly  building cumulus clouds. Early during the cumulus stage, water droplets are quite small but grow to  raindrop size as the cloud grows. The upwelling air carries the liquid water above the freezing level  creating an icing hazard. As the raindrops grow still heavier, they fall. The cold rain drags air with it  creating a cold downdraft coexisting with the updraft; the cell has reached the mature stage.  Precipitation beginning to fall from the cloud base is your signal that a downdraft has developed and  a cell has entered the mature stage. Cold rain in the downdraft retards compressional heating, and  the downdraft remains cooler than surrounding air. Therefore, its downward speed is accelerated  and may exceed 2,500 feet per minute. The down rushing air spreads outward at the surface as  shown in  figure 3-4  producing strong, gusty surface winds, a sharp temperature drop, and a rapid  rise in pressure. The surface wind surge is a "plow wind" and its leading edge is the "first gust."  Meanwhile, updrafts reach a maximum with speeds possibly exceeding 6,000 feet per minute.  Updrafts and downdrafts in close proximity create strong vertical shear and a very turbulent  environment. All thunderstorm hazards reach their greatest intensity during the mature stage.  Downdrafts characterize the dissipating stage of the thunderstorm cell as shown in  figure 3-4  and  the storm dies rapidly. When rain has ended and downdrafts have abated, the dissipating stage 'is  complete. When all cells of the thunderstorm have completed this stage, only harmless cloud  remnants remain.  The Dissipating Stage  The Mature Stage  Figure 3-4.  Life cycle of a thunderstorm. Remote Pilot – Small Unmanned Aircraft Systems Study Guide  27 