“UFO clouds” are a delightful (if somewhat ominous-looking) natural phenomenon

An occasional sight near mountain tops.
An occasional sight near mountain tops.
Image: Wikimedia Commons
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There is a gorgeous photograph of a lenticular cloud over Mount Rainier circulating on Twitter today. The photo was taken on Oct 15 at sunset in Seattle, according to @earthskyscience, the account that first shared it.

EarthSky, a media group based in Austin, Texas, tweets images of natural phenomena and the sky, plus news and research related to planetary science. Click on the link in the @earthskyscience tweet, embedded above, to see 20 more photos of these clouds at

For obvious reasons, lenticular clouds are sometimes called UFO clouds, saucer clouds, or cloud caps.

They’re not exactly rare. A well-known adage in the Seattle area is that when Mount Rainier “wears a hat,” rain is sure to follow. A 2010 blog post by Scott Sistek, a local meteorologist, said: “It’s usually a sign of rain within 24 hours because typically the moist flow that precedes a storm around here is the perfect set up for these clouds.”

The scientific name for a cloud, or clouds, like this is Altocumulus lenticularis. They are also referred to as wave clouds, because they’re generated by gravity waves in the atmosphere. According to NOAA, these develop “when relatively stable, fast moving air is forced up and over a topographic barrier,” like a mountain. “This deflection creates a gravity wave downwind of the topographic barrier not unlike a wave you might generate by throwing a pebble into a pond.”

Over a mountain range, which has multiple “speed bumps,” wave clouds might appear like this:

Over a single summit, the effect is more dramatic.