Mountain Wave Turbulence

AVIATION WEATHER

CHAPTER FIVE

Mountain Wave Turbulence

When strong winds blow approximately perpendicular to a mountain range, the resulting

turbulence can be severe. Associated areas of steady updrafts and downdrafts may extend to

heights from 2 to 20 times the height of the mountain peaks. When the air is stable, large waves

tend to form on the lee side of the mountains and extend up to the lower stratosphere for a

distance of up to 300 miles or more downwind.

Figure 5-4 Mountain Wave Turbulence

These are referred to as standing waves or mountain waves, and may or may not be accompanied

by turbulence (Figure 5-4). Pilots, especially glider pilots, have reported the flow in these waves

is often remarkably smooth. Others have reported severe turbulence.

Even though mountain wave turbulence may be present, when the airflow begins to move up the

windward side of the mountain, it is usually fairly smooth as the orographic lifting imparts the

vertical component to the motion of the air. The wind speed gradually increases, reaching a

maximum near the peak of the mountain. Past the peak, the air naturally flows down the leeward

side, completing one cycle of oscillation and setting up the standing wave pattern of the

mountain wave turbulence. Downwind, perhaps five to ten miles from the peak, the airflow

begins to ascend again, where the rotor or lenticular clouds may appear. Additional waves,

generally less intense than the primary wave, may form farther downwind. Note in Figures 5-4

and 5-5 the mountains are on the left and the wind is flowing from left to right.

While lenticular, cap, and rotor clouds are usually present to warn aircrews of mountain wave

activity, it is possible for wave action to take place when the air is too dry to form clouds,

producing CAT. Still, cloud forms particular to wave action provide the best means of

identifying possible turbulence, aside from weather forecasts and PIREPs. Although the

lenticular clouds in Figure 5-5 are smooth in contour, they may be quite ragged when the airflow

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds

5-7