Geography, ocean currents and weather patterns typical for this time of year combine to cause the gray and dreary Southern California phenomenon we call “June Gloom.”

Sometimes the phenomenon occurs a little earlier and we call it “May Gray.” Sometimes, occurrences of similar conditions arrive later in the season and are referred to as “No-Sky July” and “Fogust.”

However, the weather pattern remains mostly confined to the late spring and early summer months.

Here’s a look at how the phenomenon brings gray mornings and occasional drizzle to the coast.

1. Weather systems converge off the coast

Low pressure in the Gulf of Alaska and the Pacific Northwest promotes strong westerly to northwesterly winds along the West Coast, while high pressure lies to the east of California.

The California Current, which flows from north to south along the coast, and upwelling caused by strong winds, ensure that the waters off California remain relatively cool.

(Paul Duginski)

2. Heavy marine layer meets a cool onshore breeze

Those cool coastal waters produce a heavy, moist marine layer. An onshore breeze throughout most of May and June pushes that marine layer inland.

(Paul Duginski)

3. Catalina eddy pays a visit

Northwest winds slow down south of Point Conception and turn toward the Southern California coast during the morning hours. A counterclockwise spin called a Catalina eddy develops.

(Paul Duginski)

4. Low clouds and drizzle

This eddy behaves much like a low-pressure system, and its air becomes moist with water vapor. The moist air continues to rise and condenses into low clouds. We may start to see drizzle.

(Paul Duginski)

5. Putting a lid on it

Meanwhile, the moist, low-hanging clouds become trapped beneath an inversion layer of warmer air, which acts like a lid or cap.

This lid may sit at about 3,500 feet. It can become stronger because of the difference in temperature between air above it and below it, or the warmer inland temperatures during May and June.

Depending on the depth of the marine layer, the clouds usually dissipate as the day progresses.

(Paul Duginski)

6. Gray mornings give way to blue summer skies

Generally, by July, high pressure builds over Southern California and the winds out of the Pacific Northwest over the ocean diminish. This change helps to make the inversion — or lid — shallower or eliminate the marine layer altogether.

(Paul Duginski)

Sources: Eric Boldt, National Weather Service Los Angeles/Oxnard; Times reporting

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