Turbulence is one of the most unpredictable of all the
weather phenomena that are of significance to pilots. Turbulence is an
irregular motion of the air resulting from eddies and vertical currents. It may
be as insignificant as a few annoying bumps or severe enough to momentarily
throw an airplane out of control or to cause structural damage. Turbulence is
associated with fronts, wind shear, thunderstorms, etc. Continue...
Mechanical Turbulence. Friction between the air and the
ground, especially irregular terrain and man-made obstacles, causes eddies and
therefore turbulence in the lower levels. The intensity of this eddy motion
depends on the strength of the surface wind, the nature of the surface and the
stability of the air. The stronger the wind speed (generally, a surface wind of
20 knots or higher is required for significant turbulence), the rougher the
terrain and the more unstable the air, the greater will be the turbulence. Of
these factors that affect the formation of turbulence, stability is the most
important. If the air is being heated from below, the vertical motion will be
more vigorous and extensive and the choppiness more pronounced. In unstable air,
eddies tend to grow in size, In stable air they tend not to grow in size but do
dissipate more slowly.
Mountain waves are turbulent eddies that are
found downwind from mountain ridges. They are caused by and are therefore
stationary with respect to the mountain ridges. Mountain waves produce some of
the most severe turbulence associated with mechanical agencies. NOTE: Stability
of the lower troposphere above and to the lee of the mountain is critical
(i.e., the most intense turbulence is associated with stable air above and to
the lee of the mountain barrier).
Credit: weather.gov
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