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

What's that flying saucer cloud over Mt. Rainier?
Photo of lenticular clouds over Mt. Rainier on Dec. 5, 2008. (Photo courtesy: Tim Thompson)
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 cloud, known as a "lenticular cloud" is formed when you have three ingredients: Warm, moist air that is just on the cusp of saturation, laminar flow (when you have winds constant with height -- as in little to no turbulence or shear) and something big to get in the way, like, say, the region's tallest mountain.

When the air flows over the mountain, it will create waves downstream where the air is now going up and down, and up, and down -- like ripples on a pond or waves on the ocean. When the air goes up, it cools a little bit and when conditions are on the cusp of saturation, that slight cooling is enough to create a cloud. When the air sinks back down again, an opposite drying effect occurs and the cloud disappears.

While to us it might look like the clouds are floating in place, in fact, the air is streaming through the cloud as it hovers there -- the cloud is just showcasing the right spot in the atmosphere where the air is undergoing its lift and sink. Sometimes this occurs right over the summit, giving the mountain a hat. Other times, it's just downstream.

To get the "stack of pancakes" look, you have this effect happening at multiple layers. Watch closely in the video and you can see times where layers disappear and then magically reappear.

Here is another amazing video showing their formation:

To locals, the clouds are a sign that rain is on the way -- usually within 24 hours. That's because that needed moist air with laminar flow usually occurs in the hours preceding a weather system. Think of it as Rainier unfurling its umbrella! :)

Other stories about lenticular clouds