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.