upright=1.2|thumb|June Gloom conditions prevailing at [[Seal Beach in late morning, June 2013.]]
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
upright=1.3|thumb|Satellite image of a strong June Gloom day, showing marine layer clouds covering the coastal regions of the counties of Santa Barbara, Ventura, Los Angeles, Orange, and San Diego. The San Fernando Valley and San Gabriel Valley also are covered, with clouds spilling inland as far eastward as San Bernardino. A [[Catalina eddy is visible near the center.]]
A typical June Gloom morning consists of marine stratus clouds covering the coast of southern California, 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. 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 inversion layer is crucial to the formation of the marine stratus that produce June Gloom. Compression and warming of air sinking out of the North Pacific High-pressure system (which is strongest during the summer) meets with the rising, cooling air from the sea surface, producing a very stable layer of air that caps the cool air from rising any further. 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.