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Why you might get mid-morning bumps

Updated: May 1, 2019

Every once in a while you will see a pilot weather report of moderate or even severe turbulence one to two thousand feet above the ground with no obvious explanation of why.  It's in the mid-morning hours, the winds aloft are not all that remarkable, there are no mountains or convection nearby and not too much higher the air is extremely smooth.  So what might be going on? It might just be a somewhat common event called overturning.

Above is an example of such a pilot weather report.  At 1454Z (9:43 a.m. CDT), the pilot of a Cessna Conquest II reported severe turbulence at 2,400 feet MSL and below (elevation in that area is about 800 feet ASL).  That's about the time and altitude that overturning of the atmosphere is likely.  In order to understand overturning, you have understand how the boundary layer changes between the overnight to afternoon hours.

As the sun sets, heat is released in the form of long-wave radiation, especially when the air near the surface is calm and the sky is clear.  This is called radiational cooling.