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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