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Have you ever seen the "cap" over Mount Rainier?

Lenticular clouds, sometimes called "cap clouds," form over mountain peaks when moisture begins to increase in the upper levels of the atmosphere.

Lenticular clouds, or standing wave clouds, are associated with waves in the atmosphere. These develop when fast moving air aloft is forced up over a physical obstacle, such as a mountain range or volcano.

Once it’s forced up, the deflection generates a wave in the atmosphere downwind of the obstacle, just like waves in the water. Clouds develop within the crest of these mountain waves when the air condenses.

The condensation occurs when moisture increases in the upper levels of the atmosphere. That’s why we tend to associate lenticular clouds to rain in the forecast. The wind moves rapidly through these clouds, but they appear to remain stationary. What’s happening is these clouds are continually forming and dissipating around the wave’s crest. That’s why it looks like the cloud is hovering in place.

When you see lenticular clouds forming, you can bet that rain will be in the forecast within a couple of days.