June gloom

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. While it is most common in the month of June, it can occur in surrounding months, giving rise to other colloquialisms, such as "May Gray", "No Sky July", and, rarely, "Fogust". Low-altitude stratus clouds form over the cool water of the California Current, and spread overnight into the coastal regions of Southern 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 gives way to sunny skies. On a strong June Gloom day, the clouds and fog may extend inland to the valleys and Inland Empire and may persist into the mid-afternoon or evening.

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.

May and June together are usually the cloudiest months in coastal Southern 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.

...
Wikipedia