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Turbulence above thunderstorms

Updated: May 1, 2019

Most pilots reading this will never fly their personal aircraft above a thunderstorm.  However, if you get the opportunity to fly in a jet, turboprop or turbocharged aircraft, you just may be able to climb over some lower-topped convection.  Because of the turbulence hazard in the clear air above and surrounding thunderstorms, in an earlier version of the Aeronautical Information Manual (AIM), the FAA suggested that pilots "avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo," and "clear the top of a known or suspected severe thunderstorm by at least 1,000 feet altitude for each 10 knots of wind speed at the cloud top." Note that this statement does NOT appear in the Thunderstorms FAA Advisory Circular AC 00-24C or the previous version AC 00-24B.

Scientists at the Research Applications Laboratory (RAL) have investigated aircraft encounters with turbulence above thunderstorms and find that the FAA guidance in the AIM is a bit naive.  One of these incidents occurred on 10 July 1997 over Dickinson, North Dakota.  A commercial turbojet aircraft encountered severe turbulence as it was negotiating a path through a number of scattered thunderstorms.  At the time of the encounter it was passing directly over a developing deep convective cloud.  In this incident, 22 passengers sustained minor injuries and the aircraft sustained enough damage to cause it to make a precautionary landing at Denver, Colorado. This aircraft encountered what is referred to as convection induced turbulence (CIT).  This type of turbulence is common enough that the CIT acronym is well known within the commercial aviation community; it is used to describe turbulence in the clear air either above the thunderstorm top, under the anvil, or near the lateral visible boundaries.