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In-cloud versus out-of-cloud turbulence

Updated: May 1, 2019

While most pilots can tolerate some turbulence, many are looking for a smooth ride. In other words, you are unlikely to hear any pilot complain when the ride is glassy smooth.  So, is there an altitude that yields the least amount of turbulence, on average?     

Of course, on any particular day, there's no easy answer.  Any altitude could create a smooth ride or rough ride depending on several variables.  Some turbulence is very predictable and others not so predictable, but staying out of the cloud boundary is one of the very best ways to avoid  encounters with dangerous turbulence.  So, it is important to separate turbulence into two categories, namely, in-cloud turbulence and out-of-cloud turbulence.

Here's an interesting graph (shown above) from a study done by researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado that depicts the altitudes where moderate or greater (MOG) turbulence was reported in relation to being in-cloud or out-of-cloud. Over 650,000 pilot reports (PIREPs) were used in the study over a three year period. Since most pilots don't specify whether turbulence was in-cloud or out-of-cloud, those that fell into the MOG turbulence were compared to other datasets to determine if the turbulence report to distinguish between the two.

The yellow line represents those turbulence reports that were deemed to be in clear air (out-of-cloud). And the cyan line represents turbulence reports that were deemed to be in-cloud. Based on this study, in clear air, turbulence is most frequent at upper and lower levels of the atmosphere while mid-level turbulence is often in-cloud.

We often experience two forms of turbulence when we are within the planetary boundary layer (first 5,000 feet or so). This includes both thermal and mechanical turbulence and it's typically experienced in cloud-free air. Once you climb out of the boundary layer, you will limit the effects of the surface eliminating that form of turbulence. However, the bases of convective clouds (cumuliform) often start at several thousand feet above the surface where the air is ofte