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 often smooth out-of-cloud, but MOG within the cumuliform clouds. These clouds can often extend upward into the upper teens or lower flight levels without seeming like a hazardous threat.
The peak at the top of the diagram of clear air reports are of out-of-cloud reports are turbulence reports associated with the jet stream and more specifically, interaction with the tropopause. So, if you are relying on averages, flight outside of the cloud boundaries between 8,000 to 12,000 feet will likely minimize your exposure to MOG turbulence.
When making a pilot report for turbulence, be sure to include an accurate location as well as an indication of whether the turbulence you are reporting is in-cloud or out-of-cloud. Try to avoid reports such as "light to moderate turbulence at 6,500 feet 20 northeast of the XYZ V-O-R." Instead, report your exact radial and distance and whether or not you were in a cloud at the time. Detailed remarks are always appreciated whether they are used by a pilot or some researcher five years later.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS meteorologist