The marine layer is an expansive but shallow layer of stratus clouds that envelop coastal California in the late spring and summer. Sometimes the gloom can last weeks or off and on all season. It is a phenomenon found most intensely in the United States from Central to Southern California — although it occurs up and down the Pacific Coast from Washington state to the Baja Peninsula in Mexico. The clouds form over the cold ocean and then are blown inland by the prevailing westerly winds or by a sea breeze.

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Crazy temperature differentials can develop between the foggy coast and sunny inland areas not engulfed by the sea of clouds. Earlier this month, there was a 50-degree temperature difference in San Luis Obispo, Calif., between ocean communities and valley towns to the east. One reporter described a drive she took that day from inland Templeton to shore-side Cambria — about 20 miles as the crow flies — as “thermal whiplash.”

The marine layer forms as the result of a unique confluence of global atmospheric circulations, ocean currents and coastal geography. Critically important is a strong temperature inversion that sets up at the interface of the cold Pacific Ocean and hot surface air. For folks who have never dipped their toes in the Pacific, the air may be warm but the water is frigid.

The Hadley Cell is a planetary circulation of air that rises at the equator, moves poleward at very high altitudes (35,000 to 50,000 feet) and then sinks at about 30N or 30S of the equator. Locally, this puts California under a high pressure cell known as the North Pacific high-pressure system which causes air to sink and warm as it descends. This compressing warm air meets with the cool, evaporating air from the sea surface, producing a very stable layer of air that caps the cool air from rising any farther.

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“Air temperature normally decreases with height,” NOAA writes. “However, due to the cold water, the air temperature increases with height resulting in a temperature inversion. The air below the inversion is called the marine layer and is cooled to the point at which clouds form.”

In California, the necessary cold water swoops south along the coast on the Alaska Current. Summertime water temperatures typically don’t get out of the upper 50s or low 60s. For comparison, at a similar latitude, the Outer Banks of North Carolina can experience water temperatures in the mid-80s at this time of year.

This difference is a product of global oceanic circulation patterns. In general, West coasts of continents have currents transporting cold water south from the poles, and east coasts of continents have warm currents coming from the tropics. For this reason, marine layers are most common on west coasts.

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Often, the marine layer will burn off by the afternoon. But if doesn’t and the surface heating inland approaches the 90s and 100s, the marine layer can be drawn inland by the steep temperature gradient, which creates a sea breeze, slipping through mountain passes and providing relief to roasting valley communities.

The coastal geography of California includes a range of mountains that separates the cost from the inland valleys. Your altitude along the mountainous coastline will determine the impact of the marine layer. The thickness of the layer also is dependent on air pressure aloft (but at lower altitudes about 15,000 to 30,000 feet) and when there is a trough of low pressure approaching it can raise the cap, allowing the clouds to rise higher, sometimes up to 6,000 feet.

Just above the cap the air will be warm and stable. On June 4, a weather balloon launched from Vandenburg Air Force Base recorded a surface temperature at sea level of 55 degrees. When it reached 2,800 feet, the temperature had rocketed to 87 degrees. That’s a 32-degree difference in less than 3,000 vertical feet.