Thursday, October 20, 2011

Bit More About Cold Air In Rillito Wash

Jim asks a bit more: One more question, please: what inhibits the wind at your location when the airport is windy at night? Does your proximity to the Catalinas have an effect?

I've shown an expanded view of the area we're talking about above. The Rilltio Wash is the brown, sandy ribbon running east-west through the center of the photo. The lowest elevations in the photo are along the Wash. Basically, as cold air forms in the Wash, due to radiational cooling after the sun goes down it deepens into a stable pool of cold air within the Wash and immediate surroundings. Cold air is more dense (heavier) than the warmer, less dense, air above, which is where the winds are. The deeper the cold air gets, the more difficult it is for the winds in the warm air to mix down to the ground. So, the cold air in the wash can only drain slowly down the wash toward lower elevations due to gravity. Once the sun comes up, it rapidly heats away the cold pool. Often when I am walking just after sunrise, I observe the wind blowing in the upper limbs of very tall gum trees, while it is still perfectly calm and cool at the surface.

The airport is a nearly flat, open location. As radiation cooling occurs there, the colder air continually drifts off toward lower elevations, frequently allowing mixing to bring warm and windy air to the surface.

The Catalinas do not play a direct role in any of this, since surface winds here tend to be from either the northwest or southeast.

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