Unidentified Flying Cloud
The Altocumulus lenticularis cloud is an ‘orographic’ cloud formation, which means that it tends to form as winds are forced upwards to pass over mountains. The name lenticularis comes from the Latin for a lentil, but these clouds often look more like flying saucers.
When the atmosphere is stable, the wind flowing over the mountain can follow a rising and dipping path downwind of the peak. This is an invisible ‘standing wave’ of air, and when the wind contains enough moisture it can lead to a lenticularis cloud forming at the crest, like this wonderful example spotted by Pilar Quijada over Madrid, Spain.
In a steady wind, the position of the standing wave remains fixed and so the lenticularis cloud also stays in place. Its droplets form at the upwind side of the cloud where the air cools as it rises towards the wave’s crest. They race through the cloud with the wind, only to evaporate away again on the downwind side as the air sinks and warms once more. Although the droplets are zipping along, the points where they form and evaporate are fixed. So this UFO-shaped cloud just hovers in place in the lee of the peak.
Photograph © Pilar Quijada.