The Catalina eddy wind pattern, also called the "coastal

eddy Eddie or Eddy may refer to: Science and technology *Eddy (fluid dynamics), the swirling of a fluid and the reverse current created when the fluid flows past an obstacle * Eddie (text editor), a text editor originally for BeOS and now ported to Lin ...

" 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 atmos ...

phenomenon A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfried ...

that occurs in the

Southern California Bight The Southern California Bight is a 692-kilometer-long (430 mi) stretch of curved coastline that runs along the west coast of the United States and Mexico, from Point Conception in California to Punta Colonet in Baja California, plus the area o ...

, the mostly concave portion of the Southern California coast running from

Point Conception Point Conception (Chumash: ''Humqaq'') is a headland along the Gaviota Coast in southwestern Santa Barbara County, California. It is the point where the Santa Barbara Channel meets the Pacific Ocean, and as the corner between the mostly north ...

San Diego San Diego ( , ; ) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United State ...

. 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 ( nrf, Îles d'la Manche; french: îles Anglo-Normandes or ''îles de la Manche'') are an archipelago in the English Channel, off the French coast of Normandy. They include two Crown Dependencies: the Bailiwick of Jersey, ...

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 In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in ...

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 temperature inversion. The inversion itself is usually initiated by the cooling effect of the water on th ...

deepens and coastal stratus clouds 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 ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily ...

. 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 deserts causes air to rise, the resulting

pressure gradient In atmospheric science, 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 location. The p ...

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