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In-cloud versus out-of-cloud
turbulence 
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 shows the
altitudes where moderate or greater (MOG) turbulence was reported in
relation to being in-cloud or out-of-clouds. Click here to view a larger image.
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-clouds 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.
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