What is 'June gloom' and how does it affect Southern California?

By Kyle Elliott, AccuWeather meteorologist

The phenomenon known as "June gloom" plays a major role in altering the climate of coastal regions of Southern California when compared to areas just a few dozen miles farther inland.

June gloom refers to a band of low clouds and fog, known as the marine layer, that develops on an almost daily occurrence within a few miles of the southern California coastline during the overnight and morning hours.

"June gloom is so named as June can be one of the months that tends to have more persistent marine clouds than any other month," AccuWeather Western U.S. Weather Expert Ken Clark said.

“This phenomenon can really occur any month, especially from May through September, but it’s the persistence that is why we talk about this during the month of June," he added.

The marine layer is created by rising air over the hot, desert regions of the Southwest. This rising air creates a vacuum. Cool, moist air is then drawn in from the Pacific Ocean along the coastline of California by a subtle breeze.

This cool, moist ocean flow leads to the formation of low clouds once the sun goes down and temperatures begin to fall. When the temperature dips to a certain point but the amount of moisture in the air holds steady, the humidity rises and clouds begin to form.

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Once humidity reaches its maximum level, these clouds can thicken and lower, and fog can also form.

It is when the temperature difference between the ground and areas higher up in the atmosphere is greatest that the marine layer takes the longest to burn off.

"During the late morning and afternoon on most days, sunshine above the clouds helps to drag drier air higher into the atmosphere down to the surface, thus scouring out the clouds," AccuWeather Senior Meteorologist Alex Sosnowski said.

Once this process reverses after sundown, the marine layer tends to return and thicken, Sosnowski said.

"The marine layer can lead to huge differences in temperature between the coast to areas farther inland due to the delayed clearing at the coast, and it’s not uncommon to have it cloudy and in the 60s at the beaches while it’s in the 80s or even 90s only 10-20 miles inland," according to Clark.

For residents that live farther inland and want to escape the scorching heat, June gloom exists as an option to beat the heat and escape to the much cooler beaches. Once the clouds burn off during the afternoon, temperatures typically still get high enough (into the 60s and 70s) to enjoy the beach.

The marine layer helps to greatly moderate the climate along the coastline of Southern California. The average high in Los Angeles during the month of June remains within a couple of degrees of 70, and the average low hovers around 60.

About 50 miles farther inland, the average high in Riverside reaches the lower 90s by month's end with lows in the upper 50s.