Time lapse video shows great illustration of how Mt. Rainier 'hat' cloud works
You've probably seen it dozens of times if you've lived around here for a while -- the hat/cap cloud that sometimes hangs out around or over Mt. Rainier. But a time lapse video from a local photographer gives a great illustration of how its formed.
The cloud, known as a "lenticular cloud" is created 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.
Luke Meyers, who has a great view of the mountain, keeps a near constant watch on the summit and is ready when interesting clouds take over. His camera was rolling on May 20 when another lenticular clouds formed.
You can see how the air lifts as it approaches the summit and forms a clouds, which then floats up and over the summit, then down the leeward side where it dries out and "disappears."
Sometimes, the effect happens downwind of the mountain summit and can make for spectacular spooky sights of alien-like clouds as lenticular clouds stack atop each other:
Locals know the cloud is usually a sign of incoming rain within 24 hours or so as the conditions that bring us lenticular clouds frequently occur as the atmosphere moistens up ahead of an approaching rain event.