Cloudspotter Tom Glenn had a close encounter with what looked like a UFO while soaring a glider at 4,270 metres (14,000 feet) over Mount Palomar, California, US.

It was, in fact, an Altocumulus lenticularis cloud, a formation born in the rising and dipping airflows of stable weather over hills and mountains. Pilots of powered aircraft tend to avoid lenticularis clouds, as they can easily find themselves caught in the region of descending air currents, often accompanied by severe turbulence. But skilled glider pilots like Tom seek them out. They read Altocumulus lenticularis to know where the invisible mountain airflows are rising and offering them lift.

When you’re reliant on nothing but the air under your wings for power, you make sure there’s nothing unidentified about these flying saucers of moisture.