June Gloom
Coastal California's seasonal marine layer brings gray skies and cool air.
June Gloom is a Southern California term for a late spring and early summer weather pattern marked by overcast skies, cool temperatures, and low-altitude stratus clouds. It forms when the marine layer spreads overnight from the cool California Current into coastal regions, often accompanied by fog and drizzle. The phenomenon is notable for its regularity in May and June and its sensitivity to ocean temperature patterns such as El Niño and La Niña.
Quick Facts
- Primary months
- May and June
- Cloud layer depth
- 500 to 2,000 m / 6,562 ft
- Strengthening factor
- La Niña
- Weakening factor
- El Niño
- Historical name
- high fog
Facts from the source article.
Description
A typical June Gloom morning features marine stratus clouds covering the southern California coast, extending inland to varying distances. On strong days, the clouds and fog may reach the San Francisco Bay Area, penetrate valleys such as the Salinas Valley, or spread into the Inland Empire. The clouds move in after midnight and usually dissipate by late morning, but during a heavy season they can persist into the afternoon or all day. The air is often saturated, with fog and light mist or drizzle most common near the furthest inland extent. Solar heating typically evaporates the clouds by early afternoon. The effect is strongest at the coast and weakens inland. On strong mornings, clouds can fill the Los Angeles Basin and spill into the San Fernando, San Gabriel, and Santa Clarita valleys, and even the Inland Empire. On weaker mornings, only the beach cities within a mile or two of the coast are affected.
Climate effects
June Gloom requires a specific combination of atmospheric and oceanic conditions that usually align only around May and June. These include the marine layer, an atmospheric inversion from subsiding high-pressure air, and cool ocean water off the coast. The Catalina eddy enhances the pattern. May and June are the cloudiest months in coastal southern California, with San Diego averaging only 59% and 58% sunny days respectively. The number of gloomy days varies yearly. Warmer ocean temperatures (El Niño) tend to reduce gray days, while cooler temperatures (La Niña) usually produce more gray. Mean monthly sunshine hours drop noticeably in May and June. Some Californians report symptoms consistent with seasonal affective disorder during June Gloom, though evidence is limited. In the early 20th century it was called high fog; a long season extending into summer is known as Summer Bummer and can negatively affect coastal tourism.
Formation
The low-altitude stratus clouds form over the ocean when wind mixes moisture from the surface into the air. The air cools and expands as it rises, increasing relative humidity until it reaches 100%, at which point water vapor condenses into droplets. The stable top of the marine layer, created by a temperature inversion, prevents dry warm air from above from mixing with the stratus, confining the clouds to a vertical band of 500 to 2000 m. The inversion layer forms when air sinking from the North Pacific High-pressure system compresses and warms, meeting rising cool air from the sea surface. This produces a stable cap. The cool California Current, flowing from the Gulf of Alaska, enhances the temperature contrast. A stronger inversion—with greater temperature difference—produces deeper, longer-lasting clouds. Upwelling of colder water during La Niña strengthens the effect. Prevailing westerly winds push the clouds inland until they encounter coastal mountains at or above the cloud altitude, which block further progress. Foothill regions experience the thickest fog and drizzle.
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