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7. High-Level Mechanical Turbulence

a. The air flow becomes very disturbed when wind blows over long lines of mountains.
The wind blowing up the slope is usually relatively smooth if the air is stable. On the downwind
side the air falls rapidly down the slope. This produces strong downdrafts and causes the air
to be very turbulent. When air passing over the line of mountains has sufficient water vapor
to produce clouds, lenticular and rotor clouds form downwind from the mountains and
indicate severe turbulence. If water vapor is not sufficient to form clouds, severe turbulence
can exist even though the characteristic clouds are not present.

b. When winds faster than about 50 knots blow directly across a line of mountains, the
resulting turbulence may be extreme. Areas of continuous updrafts and downdrafts can
extend many times higher than the elevation of the mountain tops. When these conditions
occur the air cutrents make regular upward and downward movements called waves in the
downwind areas. The waves sometimes extend upward beyond the tropopause. They are
called standing waves when they tend to continue in the same location over periods of time.