What causes east winds? A high-pressure (dense) pool of cold air settles in over the Great Basin, which is east and north of Lake Tahoe. When there is a low-pressure area over Southern California, it sucks the dry, heavy, cold air from the Great Basin down into the southland. (Air moves from high-pressure areas to lower-pressure areas.)

The cold air pours like fluid westward over the Sierra range. As it descends down the west slope of our mountains toward the Pacific, the air compresses, warms, loses any moisture it has, and often wreaks havoc in Southern California in the form of a gusty, damaging wind. Residents down south in the coastal valleys fear such east "Santa Ana" winds; they are extremely dry and often a serious wildfire threat. This last week parts of the southland experienced fires and power outages due to the Santa Ana.

Sometimes we get a sub-freezing east wind here at Lake Tahoe. It drives waves onto the shores of the North and West Shores and cool things happen. Piers and bushes end up ice coated. See the pictures attached of a bush on the Tahoe City Commons Beach and rime-coated trees at 9,000 feet on Mt. Rose.



"Rime" is formed at sub-freezing temperatures when super-cooled, liquid cloud droplets are blown by the wind against solid objects like trees. The droplets freeze on contact with the objects.

Mark McLaughlin is a weather historian who lives on the North Shore of Lake Tahoe.