Wednesday, August 24, 2011

Chubasco? Not quite.

A chubasco is a sudden wind that comes down out of the mountains in the summer in the Sea of Cortez.  Is it a katabatic wind?  Maybe Cliff Mass knows for sure; I certainly don't.

Here in Seattle we don't exactly have a chubasco, but we do have a predictable, thermally-driven wind that shows up every evening in the summer when the real, prevailing winds don't disturb conditions too much.

As the land heats all day long, the air above it rises, creating low pressure over the land.  But over the Sound (water temp 57° today), the air doesn't really heat at all.  This means that it is dense, relative to the attenuated air over the land.  By evening, a river of dense, cool air is flowing down the Sound to the south, spilling into all the tributaries along the way and filling those low pressure areas.  Here on the Seattle waterfront, it is a predictable effect, resulting in a northerly that builds, starting right about dinner time, to as much as 20 kt.  Then as the sun sets and the land cools, the wind lays.

Out here on the end of G dock, it is natural air conditioning.

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