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June Gloom is a southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. June Gloom in southern California is caused by the marine layer effect common to the West Coast, and is enhanced by the Catalina eddy local to southern California. May and June together are usually the cloudiest months in coastal Southern California. June Gloom has other colloquial names if the same weather pattern occurs in May, July, or August. June Gloom is stronger in years associated with a La Niña, and weaker or nonexistent in years with an El Niño. This weather pattern occurs in other parts of the world where climates and conditions are similar. Scientists study the cloud fields that make up June Gloom to increase understanding of cloud behavior at the onset of drizzle and precipitation.
A typical June Gloom day consists of marine stratus clouds covering the coast of southern California, extending a varying distance inland depending on the strength of the June Gloom effect each day. The fog and clouds, which are formed by the marine layer, move in at night, usually after midnight, and typically dissipate in the late morning, giving way to clear, sunny skies. During a heavy June Gloom season, the condition may persist into the afternoon, or even all day during an exceptionally strong event. If the air is saturated with moisture, fog also may develop with June Gloom. Early mornings during June Gloom are typically foggy, with frequent light mist and occasional drizzle. Fog and drizzle normally are found near the furthest inland extent of the gloom, where the cloud deck is closest to the ground. The fog recedes and reveals low clouds by mid-to-late morning; by late morning to early afternoon, solar heating usually is sufficient to evaporate the clouds altogether. The phenomenon forms earliest and lasts longest at the coast, with later formation and earlier dissipation in areas further inland. When the marine layer is strong and deep, clouds can fill the Los Angeles Basin and spill over into the San Fernando Valley and San Gabriel Valley, even extending into the Santa Clarita Valley on exceptionally strong June Gloom mornings. If the condition is not as strong, the Basin may be filled while the valleys may be clear. It is not uncommon for motorists to drive over the Sepulveda Pass from the clear, sunny San Fernando Valley and plunge into a cloudy, fog-filled Los Angeles. On a weak June Gloom morning, the clouds and fog may only be present within a mile or two of the coastline, affecting only the beach cities.
The months of May and June are typically the cloudiest months of the year in coastal southern California, having only 59% and 58% sunny days, respectively, on average. The number of days in May and June that are "gloomy" vary from year to year. Anomalies in sea surface temperature can be used to forecast the number and intensity of June Gloom days. Years with warmer ocean temperatures, referred to as El Niño, may result in fewer gray days in May and June. Cooler ocean temperatures, associated with La Niña, usually foretell a more gray period.
June Gloom has been reported by some Californians to bring on symptoms consistent with seasonal affective disorder, although this is not well-supported by evidence. However, the normally-very-sunny Los Angeles climate also is home to people who thrive during the brief seasonal respite the gloom provides from the unending sunshine and clear skies.
June Gloom has other names in southern California if it occurs in other months. These include May Gray if it begins early, and No-sky July or Fogust if it continues past June. In the early 20th century, this phenomenon was sometimes known as the high fog. A long June Gloom season, extending late into the summer, is known as Summer Bummer. The negative effects of a long June Gloom on the coastal California tourism industry is often reported in the local news media. The phenomenon can be especially disorienting to visitors from inland areas who, coming from the summer heat, would not expect cool temperatures and clouds and fog at the beach.
The condition is prevalent in many parts of the world where an offshore marine layer of stratus or stratocumulus clouds is common, such as the western coasts of continents—particularly off Peru, Namibia, Western Australia, atlantic Sahara and Northern California, particularly San Francisco. Such cloud systems are persistent year-round off the coast; in certain seasons they move ashore and create the cloudy, cool effect on land. These places typically are located in a Mediterranean climate (Köppen climate classification Csa), but the effect occurs in other climate zones as well, where conditions are favorable. San Francisco has fog throughout much of the year, common through some of Northern California. Central California also has areas with cooler weather as the result of the fog through much of the year.
A similar phenomenon can occur in the Pacific Northwest between May and early July, though the phrase "June Gloom" is not nearly as commonly heard as it is in California. In the Pacific Northwest, it is often referred to as "June-uary". As locations east of the Cascade Mountains rapidly heat up in late spring, the resultant pressure gradient pulls cool marine air onshore, over the coastal mountains, and into the Willamette Valley of Oregon, as well as the non-coastal parts of southwestern Washington. At night the already cool and moist marine layer becomes even cooler, fueling the formation of low clouds. During late spring and early summer, it is quite common for cities like Portland to be completely overcast in the morning and remain as such until early afternoon, at which time the clouds dissipate and the sky becomes mostly clear.
As summer progresses, the marine layer typically gets shallower and has some difficulty crossing the Coast Range. While stratus cloud decks can occasionally reach the interior valleys in July and August, they are very often confined to the coastline during this time.
Researchers have discovered that the cloud fields forming June Gloom and related phenomena from other west-coast marine-influenced climates are excellent places to find and study actinoform clouds. These clouds have been found to be present more often than expected in common stratocumulus layers. These clouds are persistent year-round off the coast, but are only drawn inland during June Gloom events and related phenomena elsewhere in the world. Observations suggest that when marine stratus is present alone, drizzle is minimized. However, scientists believe that the presence of actinoform clouds within the marine stratus is indicative of an increase in drizzle and the onset of precipitation. Observation and computer modeling have shown that the shape of the cloud fields actually rearrange themselves when the clouds start to rain.