UFO or lenticular clouds form when stable, moist air travels over mountains and creates standing waves in the atmosphere.
As the air cools, it then condenses into flat clouds, often stacking up like UFOs or pancakes.
Lenticular clouds often signify strong winds on the ground below.
The Met Office said: “When air blows across a mountain range, in certain circumstances, it can set up a train of large standing waves in the air downstream, rather like ripples forming in a river when water flows over an obstruction.
“If there is enough moisture in the air, the rising motion of the wave will cause water vapour to condense, forming the unique appearance of lenticular clouds.”
The Met Office says pilots avoid flying near the lenticular clouds as they can cause turbulence but some experienced glider pilots like them as it is a visual representation of where the air is rising.