The Catalina eddy wind pattern, also called the "coastal eddy" or "marine layer," is a localized

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...

phenomenon A phenomenon ( phenomena), sometimes spelled phaenomenon, is an observable Event (philosophy), event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be ...

that occurs in the Southern California Bight, the mostly concave portion of the Southern California coast running from Point Conception to

San Diego San Diego ( , ) is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. With a population of over 1.4 million, it is the List of United States cities by population, eighth-most populous city in t ...

. The Catalina eddy leads to June Gloom, which is so much a part of the late spring and early summer weather in Southern California. The eddy is named for Santa Catalina Island, one of the

Channel Islands The Channel Islands are an archipelago in the English Channel, off the French coast of Normandy. They are divided into two Crown Dependencies: the Jersey, Bailiwick of Jersey, which is the largest of the islands; and the Bailiwick of Guernsey, ...

offshore between Los Angeles and San Diego. Though the coastal marine layer can develop at any time of the year, predominantly these eddies occur between April and September with a peak in June. During these months, upper-level northwesterly flow along the California coast is directed onshore by the Channel Islands. When the flow is blocked by the mountains that ring the

Los Angeles Basin The Los Angeles Basin is a sedimentary Structural basin, basin located in Southern California, in a region known as the Peninsular Ranges. The basin is also connected to an wikt:anomalous, anomalous group of east–west trending chains of mountai ...

to the east and north, a counterclockwise vortex is created. As temperatures drop after sunset, the

marine layer A marine layer is an air mass that develops over the surface of a large body of water, such as an ocean or large lake, in the presence of a Inversion (meteorology), temperature inversion. The inversion itself is usually initiated by the cooling ...

deepens and coastal

stratus cloud Stratus clouds are low-level clouds characterized by horizontal layering with a uniform base, as opposed to convective or cumuliform clouds formed by rising thermals. The term ''stratus'' describes flat, hazy, featureless clouds at low altit ...

s thicken. While the vortex is relatively small, rarely more than 100 miles (160 km) in diameter, it can extend into inland valleys and even into the southwestern

Mojave Desert The Mojave Desert (; ; ) is a desert in the rain shadow of the southern Sierra Nevada mountains and Transverse Ranges in the Southwestern United States. Named for the Indigenous peoples of the Americas, indigenous Mohave people, it is located pr ...

. A very strong Catalina eddy can be as deep as 6000 feet (1.8 km). A Catalina eddy is rarely prolonged. As the heat over the

desert A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the la ...

s causes air to rise, the resulting

pressure gradient In hydrodynamics and hydrostatics, the pressure gradient (typically of air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular locat ...

and increase in the normal onshore winds causes the vortex to dissipate. The result is the common local weather forecast calling for "late night and early morning low clouds and fog, followed by afternoon sunshine and sea breezes."

References

*Maas, Clifford F. and Mark D. Albright, "Origin of the Catalina Eddy," Monthly Weather Review, American Meteorological Society, Volume 117, Issue 11 (November 1989)
Glossary, National Weather Service, National Oceanic and Atmospheric Administration (NOAA)
*Gales, Donald Moore, "Handbook of Wildflowers, Weeds, Wildlife and Weather of the South Bay and Palos Verdes Peninsula", 4th Ed. Published by Foldarol Company 2003, {{Cyclones Marine meteorology Climate of California Weather events in the United States