Valley Cloud

Credit Daniel Simpson cc-by – tap for original

Cloud of the Day – Valley Cloud

In my valley winters are often visited by a phenomenon called valley cloud. Those days are clear and sunny, except in the valleys. If you’re up on the slopes you’re likely to see ski bunnies in bikinis. But down in the valley the Sun is blocked by a layer of valley cloud.

Credit J.M.Garg cc-by-sa – tap for original

Those of us who grew up here don’t mind it. That low cloud is kind of like a snoozy blanket, and we snuggle in. That’s winter. We’re used to it. But people who move here from away, especially from the Prairies, don’t like it so much. On the Prairies it might get bone-cracking cold, but the sky is bright blue and the horizon meets it out near infinity. It’s bad enough that the hills make them feel hemmed in, but now this persistent cloud is blocking the sky and dimming the light. It’s no wonder they look haunted.

Valley cloud develops under a stalled upper ridge of high pressure. Instead of moving generally eastward, as systems tend to do, it sits there, pressing down on the air below. Now a temperature inversion forms, with warmer air over cooler air. A stationary situation with little wind and no convection means the cool, moist air is trapped under the warmer, drier air. There will be condensation in the valleys, resulting in fog or cloud. As fog can be thought of as cloud in contact with the ground, I like to think of valley cloud as fog up in the air.