There are three basic requirements for the formation of thunderstorms: unstable air (indicated by an ambient lapse rate of more than 2 degrees C per 1000 ft), an initial updraft (caused by heating or a fast-moving front), and high moisture content (the temperature and dew point are close). Thunderstorms progress through three distinct stages: cumulus, mature, and dissipating. The initial stage is always a cumulus cloud with updrafts that may extend from the earth’s surface to several thousand feet above the visible cloud tops. A developing storm can climb at a couple of thousand feet per minute or more in extreme cases, so out-climbing a fast-developing storm in a light plane is usually impossible.

Rain commencing to fall from the base of a cumulonimbus cloud to the surface is a good indication that a thunderstorm has entered the mature stage, and it is in this stage that the thunderstorm reaches its greatest intensity with violent updrafts and downdrafts. The mature stage of a thunderstorm typically lasts 20 to 40 minutes. However, a steady-state storm can stay in the mature state for as long as 24 hours and travel as far as 1000 miles, taking in energy from behind and releasing downdrafts and gusts in its direction of movement, all with severe to extreme downdrafts, wind shear and turbulence. Squalls (wind speed of at least 15 knots for more than one minute with a peak of at least 20 knots) or gusts (wind speed of at least 10 knots) may precede the storm.

Precipitation cools the lower portion of the cloud, reducing the buoyancy of the air and effectively shutting down the “engine” of normal, limited-state thunderstorms. During this dissipating stage, updrafts cease and cold downdrafts flow out of the base of the cloud.

In case of inadvertent entry into a thunderstorm, maintain a straight course to minimize time in the cloud; turns only increase the structural stress on the aircraft. Reduce power to the turbulence penetration speed described in the POH/AFM and try to minimize additional power adjustments. Try to maintain a level attitude while allowing airspeed and altitude to fluctuate.

In-cockpit radar such as NEXRAD is good for strategic decisions but not for tactical decisions because of the 6-8 minute delay. Trying to “threat the needle” and fly between cells of thunderstorm activity is very dangerous because the cell may have moved directly into your flight path during the delay. Instead, the radar information should be used to give indicated areas of thunderstorm activity a wide berth.