MART Vol. II MO/MP - NESA - Civil Air Patrol

Figure 3-5Most cumulus clouds do not become thunderstorms, but all thunderstorms areborn as cumulus clouds. The main feature of this first stage of thunderstormdevelopment is its updraft, a large air current flowing upward from the groundthrough the chimney-like cloud. The draft can reach speeds of several thousandfeet per minute, and continue to an altitude of 40,000 feet or more. During thisperiod, small water droplets grow into raindrops as the cloud builds upward tobecome a cumulonimbus cloud.Precipitation at the earth's surface marks the mature stage of a thunderstorm.The raindrops (or ice particles) have now become so large and heavy that theupdraft can no longer support them, and they begin to fall. As they fall, theraindrops drag air with them, causing the characteristic strong down draft ofmature thunderstorms. These down drafts spread out horizontally when theyreach the surface, producing strong, gusty winds, wind shear, sharp drops intemperature (because the air was chilled at high altitudes) and a sharp rise inpressure.The mature stage of the thunderstorm is when associated hazards are mostlikely to reach maximum intensity. Micro bursts, extremely intense down drafts,can occur during this mature phase of development. Downward wind velocities inmicro bursts may reach 6,000 feet per minute, and even powerful jet aircraft mayhave insufficient power to recover prior to ground impact.As down drafts continue to spread, updrafts weaken, and the entirethunderstorm eventually becomes an area of down drafts, which characterizes thedissipating stage of the thunderstorm. During this stage, the cloud develops thecharacteristic anvil shape at the top and may take on a stratiform or layeredappearance at the bottom. Usually this stage is the longest of the three stages ofa thunderstorm's life.No thunderstorm should ever be taken lightly. During the cumulus stage,vertical growth occurs so quickly that climbing over the developing thunderstormis not possible. Flight beneath a thunderstorm, especially in the mature stage, isconsidered very foolish, due to the violent down drafts and turbulence beneaththem. Flight around them may be a possibility, but can still be dangerous. Eventhough the aircraft may be in clear air, it may encounter hail, lightning, orturbulence a significant distance from the storm's core. Thunderstorms should beavoided by at least 20 miles laterally. The safest alternative, when confronted bythunderstorms, is to land, tie the aircraft down, and wait for the storms to dissipateor move on.58