What's that flying saucer cloud over Mt. Rainier?

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By Scott Sistek

Story Published: Oct 4, 2006 at 12:47 PM PST

Story Updated: Dec 21, 2009 at 9:07 AM PST

The infamous "cap cloud" that sometimes forms over Mt. Rainier has been the source of legends and folklore for ages around here. Some say it looks like an alien spaceship is descending on the mountain's summit.

But there's nothing mystical about how it's formed.

The cap cloud (the official name is "lenticular cloud") is formed when warm, moist air runs into the surface of Mt. Rainier. The mountain's topography forces the air upward, which cools and condenses the air -- turning it into a cloud.

As the air sinks back on the other side of the mountain, it dries out and the cloud dissipates. That's why it just hangs over the summit area.

(Although it looks like it is "hanging" over the mountain, air is continually flowing over the summit.)

Locals have used the cap cloud as a sign that rainy weather is on the way, as it usually occurs with southwesterly flow in the upper atmosphere -- a usual precedent to rainy weather. However, that's not always the case.

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