Lenticular Clouds

Near mountain ranges you can sometimes see clouds form into a very particular shape, one that is easy to mistake for a UFO. When wind travelling at speed runs into a mountain, it climbs fast and curls into an eddy, and that swirl gives the cloud its smooth, lens-like form.

Not every strange cloud is lenticular, though. Other formations can fool the eye just as well, stacking and glowing in ways that look engineered rather than natural.

Knowing the trick does not make the sky any less strange. It just means that before you call something a craft, it is worth asking the wind first.