Lenticular clouds

Are stationary lens-shaped clouds that form at high altitudes, normally aligned perpendicular to the wind direction. Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. If the temperature at the crest of the wave drops to the dew point, moisture in the air may condense to form lenticular clouds. Pilots of powered aircraft tend to avoid flying near lenticular clouds, but glider pilots actively seek them out.