Lenticular clouds form when moist, warm air strike the flanks of Rainier and are deflected upward where it cools and condenses into cloud like the cap of a mushroom.
The air forms a standing wave as it streams over the mountain’s peak and descends the far side into the trough, warming and drying as it falls. The cloud remains stationary at the crest of the wave, continually resupplied by the moist air drawn from the westerly wind and the sea.
Photo attribution: Tim Thompson.
Lenticular clouds form over Mt. Rainier several times each year but rarely are they this spectacular. Because of the turbulence associated with the formation of lenticular clouds, the pilots of powered planes avoid them but sailplanes ride the wave lift to great heights and distances. Imagine riding a sailplane through such a cloud!
The only named wind in the British Isles, the Helm Wind, forms similar clouds above Cross Fell. The clouds are called the Helm Bar.