L e n t i c u l a r i s 

Lenticular or lens-shaped clouds usually form over mountainous areas. They resemble a huge lid or a tall hat hovering above the mountains. When there is more than one, lenticular clouds can look like a stack of giant dinner plates or a formation of flying saucers.

The cause is quite simple. Winds carrying air over a mountain rise up one side, cooling on the way. The moisture in the upward air flow condenses to form a cloud. When the air moves down the other side of the mountain, it warms up. The droplets in the cloud then turn back to water vapour because warm air can hold more vapour than cool air. Since winds are constantly blowing over the mountain, the clouds are continually renewed. They therefore appear to remain in position, since they mark the spot where air is cooled into condensation.

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. Under certain conditions, long strings of lenticular clouds can form at the crests of these waves. When these clouds touch one another a formation is created that is known as a wave cloud.

Viewing the pictures below, it is easy to understand why lenticular clouds are sometimes mistaken for UFOs, especially when viewed from a moving car with houses and trees hampering the view.