Imagine driving through mountain country and spotting a smooth, round, silvery shape hovering above a peak. It doesn't drift with the wind. It just sits there, like a spacecraft waiting for something. It's no wonder some people have called in sightings of flying saucers.
What they're seeing is almost always a lenticular cloud. These clouds form when moist air flows over mountains and creates invisible waves in the sky. Where the air rises to the top of a wave and cools enough, a smooth, lens-shaped cloud appears. Because the wave stays in place, the cloud does too.
Named After a Lentil
The name comes from the Latin word lenticularis, meaning "lentil-shaped." That's a good description. These clouds often look like a lens, a lentil, or a saucer, with smooth edges and a rounded top. Even some rare clouds high in the lower stratosphere, called nacreous clouds, can take on the same lens shape.
There are three main types, sorted by height above the ground. Some form at low levels, some in the middle of the sky, and some very high up among the icy cirrus clouds. Sometimes several form on top of each other, like a stack of pancakes, when layers of dry and moist air alternate overhead.
Waves You Can't See
Air flowing over the land behaves a bit like water flowing over a rock in a stream. When wind hits a mountain, it's pushed upward and then flows down the other side. If the air is stable, it keeps bouncing up and down downwind of the mountain, forming a series of standing waves. Meteorologists call these lee waves or mountain waves.
As air rises toward the crest of a wave, it expands and cools. If it cools below its dew point, the moisture condenses into a cloud. On the way back down, the air warms up again and the cloud can disappear. That's why lenticular clouds often have such neat, sharp edges.
Most clouds drift along with the wind. Wave clouds don't. They stay fixed in place relative to the mountain that creates them, which is exactly what makes them look like something hovering on purpose.
Why They Get Mistaken for UFOs
Lenticular clouds generally don't form over flat or low-lying land. So many people have simply never seen one and don't know such a cloud exists. Add the classic flying saucer shape, the smooth surface, and the eerie stillness, and it's easy to see why some observers think they've spotted something from another world.
Sometimes they even shimmer. Bright rainbow-like colors, called iridescence, can appear along their edges. That only adds to the otherworldly effect.
A Glider Pilot's Best Friend
Pilots of powered planes tend to steer clear of lenticular clouds because of turbulence and sinking air near them. Glider pilots, though, go looking for them. The rising air at the front of the cloud can carry an engineless glider to astonishing heights.
In fact, mountain waves were discovered in 1933 by two German glider pilots. As of 2020, the gliding world records for both distance, over 1,800 miles, and altitude, over 74,000 feet, were set using this kind of wave lift.
So if you ever spot a silent saucer parked over a mountaintop, grab your camera, not the phone to call the authorities. You're looking at one of the most beautiful tricks the atmosphere can play.
