June Gloom is a mainly Southern California term for a weather pattern that results in cloudy, overcast skies with cool temperatures during the late spring and early summer. While the marine layer is most common in the month of June, it can occur in surrounding months, giving rise to other colloquialisms, such as Graypril, May Gray, Junuary, No-Sky July, and Fogust. Low-altitude stratus clouds form over the cool water of the California Current, and spread overnight into the coastal regions of California. The overcast skies often are accompanied by fog and drizzle, though usually not rain. June Gloom usually clears up between mid-morning and early afternoon, depending on the strength of the marine layer and the distance of the location from the Pacific Ocean, and gives way to sunny skies. May and June together are usually the cloudiest months in coastal California. June Gloom is stronger in years associated with a La Niña, and weaker or nonexistent in years with an El Niño. This weather pattern is relatively rare, and occurs only in a few 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 precipitation.
Description
A typical June Gloom morning 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 that day. On a strong June Gloom day, the clouds and fog may cover the San Francisco Bay Area, penetrate far inland down valleys such as the Salinas Valley in central California, or extend into the Inland Empire of southern California. It's not uncommon for the layer to persist into the mid-afternoon or evening. The 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. Often, the air is saturated with moisture, and fog also develops, along 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. By late morning to early afternoon, solar heating usually is sufficient to evaporate the clouds, and the sun emerges. The phenomenon forms earliest and lasts longest at the coast, with weaker effects as it moves 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 and Inland Empire on exceptionally strong June Gloom mornings. If conditions are 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.
Climate effects A combination of atmospheric and oceanic conditions must be just right in order for June Gloom to form, and these conditions usually align only around May and June of each year. These include the marine layer effect common to the West Coast of the United States, an atmospheric inversion caused by subsidence of high-pressure air from the subtropical ridge, and sufficiently cool ocean water off the coast. The June Gloom pattern is also enhanced by the Catalina eddy local to southern California. 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 in San Diego. The number of days in May and June that are "gloomy" varies from year to year. Anomalies in sea surface temperature can be used to forecast the length and intensity of the June Gloom phenomenon in a given season. 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. The climate charts below show a clear drop in the mean monthly sunshine hours and percent possible sunshine for the months of May and June, which are the two months when the June Gloom pattern is the strongest.
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. 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.
Formation
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