Interesting Special Images from our Archives

A Catalina Eddy on 10/13/95
The Catalina eddy, named for the Catalina Islands off the coast of Los Angeles, is an occasional phenomenon of coastal southern California south of Point Conception. It sometimes forms when the wind across the region in the lower atmosphere blows from the north or northeast.

As air crosses the east-to-west oriented mountain ranges north of Santa Barbara (just east of Point Conception) and descends to the ocean, the pressure on the air increases, causing it to warm dramatically. As an indirect consequence of this warming, a region of relatively low pressure (compared to surrounding areas at the same altitude) develops in the lower atmosphere, south or southwest of the east-to-west oriented coastline. This warm, cloud-free region of relatively low pressure offshore draws cool marine air up the coast from the south. The marine air--which is often full of fog or low stratus clouds visible from space--then spirals around the low pressure center, creating the eddy.

Below is a large-scale overview of the weather pattern within which this fairly typical Catalina eddy developed, on the morning of October 13, 1995:

The sequence of images below, focusing on the southern California coastal region, provides a closer view of the eddy at two-hour intervals during its development. Each image shows weather reports from land-based stations and buoys superimposed on a visible satellite image, recorded on October 13, 1995. (All times listed are Pacific Daylight Time.) On these close-up images, note how the cloudy marine air flows first northward and then westward along the coast, toward the low pressure southwest of Point Conception. As this air encounters the Catalina Islands and other islands offshore, it flows partly around them and partly over them. The air that flows over the islands warms by compression (as described above), which sometimes completely evaporates the cloud droplets suspended in that air. The result--visible on some of the images above--is a small clear area just downwind of some islands. As the sun gradually heats the islands, foggy/cloudy air in contact with them can warm by conduction of heat, ultimately evaporating the fog/cloud droplets as well, leaving at least parts of some islands cloud-free as well.


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