Cumulus Stage (3–5 mile height)  Mature Stage (5–10 mile height)  Dissipating Stage (5–7 mile height)  32 °F  0 °C  Equilibrium level  40,000 ft.  30,000 ft.  20,000 ft.  10,000 ft.  5,000 ft.  Figure 12-28.  Life cycle of a thunderstorm.  A squall line is a narrow band of active thunderstorms. Often  it develops on or ahead of a cold front in moist, unstable  air, but it may develop in unstable air far removed from  any front. The line may be too long to detour easily and too  wide and severe to penetrate. It often contains steady-state  thunderstorms and presents the single most intense weather  hazard to aircraft. It usually forms rapidly, generally reaching  maximum intensity during the late afternoon and the first  few hours of darkness.  least 20 nautical miles (NM) since hail may fall for miles  outside of the clouds. If flying around a thunderstorm is not  an option, stay on the ground until it passes.  For a thunderstorm to form, the air must have sufficient water  vapor, an unstable lapse rate, and an initial lifting action to  start the storm process. Some storms occur at random in  unstable air, last for only an hour or two, and produce only  moderate wind gusts and rainfall. These are known as air  mass thunderstorms and are generally a result of surface  heating. Steady-state thunderstorms are associated with  weather systems. Fronts, converging winds, and troughs  aloft force upward motion spawning these storms that often  form into squall lines. In the mature stage, updrafts become  stronger and last much longer than in air mass storms, hence  the name steady state.  [Figure 12-29]  The most violent thunderstorms draw air into their cloud  bases with great vigor. If the incoming air has any initial  rotating motion, it often forms an extremely concentrated  vortex from the surface well into the cloud. Meteorologists  have estimated that wind in such a vortex can exceed 200  knots with pressure inside the vortex quite low. The strong  winds gather dust and debris and the low pressure generates  a funnel-shaped cloud extending downward from the  cumulonimbus base. If the cloud does not reach the surface,  it is a funnel cloud; if it touches a land surface, it is a tornado;  and if it touches water, it is a “waterspout.”  Knowledge of thunderstorms and the hazards associated with  them is critical to the safety of flight.  All thunderstorms have conditions that are a hazard to aviation.  These hazards occur in numerous combinations. While not  every thunderstorm contains all hazards, it is not possible to  visually determine which hazards a thunderstorm contains.  12-23  Tornadoes  Squall Line  Hazards 