Where there is lightning, there is a thunderstorm. Thunderstorms indicate
a lot of air movement, up, down, and laterally. If you are taking off from
or landing at an airport anywhere near a thunderstorm, you can expect wind
Thunderstorms can exist inside an air mass, along frontal boundaries, or out
front of a cold front in a band of weather called a squall line. Squall
line thunderstorms generally produce the most intense hazard to aircraft.
Three things are required for thunderstorm formation:
Sufficient water vapor
Unstable lapse rate
Thunderstorms have a life cycle defined by three stages of development, which
are cumulus, mature, and dissipating. The cumulus stage is dominated by a
continuous updraft. In this stage, the thunderstorm develops from a small
cumulus cloud into a towering cumulus cloud. Precipitation indicates the
mature stage of the thunderstorm. Thunderstorms reach their greatest
intensity during the mature stage. Finally, the thunderstorm dissipates.
The dissipating stage is characterized predominately through downdrafts.
A microburst is a small scale intense downdraft which spreads outward near the ground in all directions. A microburst usually builds in intensity for the first five minutes, then persists at maximum intensity for 2-4 minutes, and dissipates within about 15 minutes. Pilots encountering a microburst will experience a headwind, then an intense downdraft, and finally a severe shear to a tailwind. This translates to increased performance initially, followed by decreased aircraft performance, which could then potentially degrade to performance so poor the pilot is unable to recover before impacting terrain.
Lifting force can be provided through heating from below, frontal lifting, or mechanical lifting by terrain.
The downdrafts inside a microburst are known to consist of air descending at as much as 6,000 feet per minute.