Lenticular Clouds: The Cloud That Stays Put

A lenticular cloud usually sits a little downwind of a summit, smooth as a lens and oddly still. The wind at that height may be blowing at sixty kilometres an hour or more, yet the cloud can hold its place for an hour. What stays is the shape, not the water: air rises into the crest, cools until its moisture condenses, then sinks out of the far side and dries out again.

The mountain sets this up. Stable air pushed over a ridge starts to bob up and down downstream, the way water ripples behind a rock in a stream, and the ripples can repeat for tens of kilometres. Where a wave crest lifts air past its dew point, a cloud appears; where the trough sinks, the sky stays clear. When several moist layers lie on top of each other, the clouds stack into something like a pile of plates.

Glider pilots go looking for them, because the rising side of the wave can carry an aircraft several kilometres higher, while airline crews read the same sky as a sign of turbulence lower down. On a screen, a lenticular is easy company: one clean outline, a lot of plain sky around it, and a shape that still reads at the size of a thumbnail.