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. This process causes the air right near the surface to cool enough to produce what's called a surface-based nocturnal temperature inversion as depicted on the Skew-T log (p) diagram above. This inversion continues to deepen throughout the overnight hours until the sun starts to rise. At some point during the morning, the sun heats the ground and begins to "mix out" that nocturnal inversion layer (NIL) or a process that is called the "initial filling of the nocturnal inversion."
The morning transition can be divided into two distinct phases. After sunrise a shallow convective boundary layer (SCBL) develops below the formerly ground-based NIL.
On a summer morning with dry surface conditions and a cloudless sky, the top of the SCBL increases rapidly due to heating of the surface, the result is upward turbulent mixing and the entrainment of dry air from above. This kind of morning transition or what is called as "the rapid mixing out of the nocturnal inversion" does possibly not provide enough time for a thorough mixing of the SCBL and the reaching of an equilibrium in a slower fashion as observed during the afternoon. After about two hours the surface convection reaches the top of the inversion or the residual layer (RL) base depicted below.
Now the second, shorter phase starts as the thermals (heated air) shoot through the RL until they reach the capping inversion of the convective boundary layer (or mixed layer). Now, at the end of the transition, the well-mixed quasi-stationary daytime mixed layer begins to establish. When this second phase occurs and the air penetrates through the RL, then you get this overturning that often results in moderate or severe convective turbulence.
Think of this as releasing air a foot or two under the surface of the water in a pool. Those bubbles of air quickly rise to the top and then equalize on the surface. This is similar to the process that occurs as convective heating "breaks through" the remaining residual inversion layer causing the air to violently mix for a short period of time (normally 10 to 20 minutes). So if you just happen to be in the right place at the right time, you might get bumped around a bit during the mid-morning hours.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist