Winds and turbulence are the primary weather hazard in the mountains. Wind crossing a ridge line accelerates, producing speeds that can be double the speed reported in the Winds Aloft Forecast for the same altitude. As the air is lifted and cooled, clouds or upslope fog can form. Precipitation is much more likely on the windward side of mountains. By contrast, descending air on the leeward side of mountains is warmed and dried, reducing the chance of rain. However, this air is also very turbulent and contains downdrafts which can exceed the climb capability of light aircraft. One knot of wind blowing across a ridge typically produces 50 feet of downdraft per minute on the leeward side. So a 20 knot wind (doubled to 40 knots as it crosses the ridge line) can result in a downdraft of 2000 feet per minute.

When strong airflow over the mountains is accompanied by other conditions such high winds in the upper atmosphere and an inversion layer near the mountaintop, a phenomenon called mountain wave may occur. Oscillating updrafts and downdrafts may extend as far as 100 miles or more on the downwind side of the mountain or ridge line. Mountain wave is often indicated by the presence of lens-shaped clouds called lenticular clouds which can form just above ridge top level to as high as 40,000 feet. Areas where lenticular clouds are observed should be avoided at all cost—updrafts and downdrafts in the violent rotors that accompany them can exceed 5000 feet per minute.

In the mountains it is critical to know wind direction. Pilots should try to obtain wind reports from 3 different airports: the landing area, the bottom of the valley, and the other side. During the morning and afternoon, air heated by the sun will tend to flow up mountain valleys toward higher elevations. This phenomenon is called valley breeze and will generate a tailwind of approximately 4 knots when taking off or landing upstream. In the evening, air flow is reversed as cooling air flows down the mountain valleys toward lower elevations. This phenomenon is called mountain breeze and will generate a tailwind of approximately 8-12 knots when taking off or landing downstream.

While airborne, wind direction can be ascertained by observing the movement of cloud shadows along the ground. Lakes can also indicate wind direction. The calm part of a lake is upwind. Whitecaps form when the wind is about 10 knots and appear to move upwind (because the waves underneath them are moving downwind).