Lenticular cloud

Lenticular cloud over Campbell Mesa, Flagstaff, Arizona.

A lenticular cloud (Altocumulus lenticularis) is a type of cloud that generally forms at high altitudes. They are shaped like discs and remain roughly in the same position. Because of the way they are shaped, they have been offered as an explanation for some UFO sightings. Lenticular clouds are formed when the wind forces the air up a mountain ridge to where the air is cool enough for water vapor to condense.

Lenticular Cloud Media

Lenticular cloud over the Antarctic ice near Scott Base.
A lenticular cloud covers the summit crater of Mayon Volcano, Philippines.

References

This short article about weather can be made longer. You can help Wikipedia by adding to it.