The Marine Layer

How the marine layer affects temperatures.

Somewhat similar to the sea breeze, the marine layer also results from the interaction between air masses of different temperatures and densities. However, unlike the east coast sea breeze, which reforms almost every day due to the rise and fall of heated and cooled air in the summer atmosphere, the west coast marine layer can persist for days or weeks due to a process called a temperature inversion. This is particularly seen along the coasts of Central and Southern California.

Golden Gate bridge towers sticking up above the marine layer.

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While the eastern Gulf Stream brings warmer tropical water north, water along the west coast of North America moves south from the Gulf of Alaska, bringing much cooler surface temperatures. The water off the California coast can be as much as 30°F (17°C) lower than water at the same latitude on the east coast.

This colder water then cools the air in contact with it, increasing the air’s density. When the lower layers of air are cooled from the bottom up, the air higher up remains relatively warmer and less dense, a reverse of what is typically seen. As a result, there is no temperature or pressure imbalance to drive the convection found in a sea breeze. Instead, the air masses remain layered in what is called a temperature inversion.

The air below the inversion is the marine layer, which can cool to the point at which clouds or fog form low to the surface. A person driving across San Francisco's Golden Gate bridge in summer will commonly encounter fog and temperatures in the upper 50s to lower 60s, only to drive a few miles north to the top of Mount Tamalpais (elevation 2,571 feet/ 784 meters) and find clear skies with a temperature in the 80s or even the lower 90s. This is due to passing through the temperature inversion and above the cool marine layer.

Because of its persistence in early summer, people in Southern California often refer to it as the "May Gray" or "June Gloom". However, this can occur near any large body of water in which the water temperature is significantly colder than the air moving over it, such as the Great Lakes.

The depth of the marine layer depends upon the large-scale weather patterns that pass high overhead. High pressure systems located at elevations from 15,000 to 30,000 feet (4,500 to 9,000 meters) will squash the marine layer down to the earth's surface. If the downward force of air is very strong, the marine layer will be very shallow, with low clouds and foggy weather confined to the beaches; very warm, sunny conditions will begin just a mile or so inland.

A decrease in air pressure above the marine layer allows it to rise and deepen. Near the beach, the fog lifts into a low cloud layer, while the leading edge of the marine layer extends farther inland, bringing fog with it.

Further lifting of the marine layer will allow cooler marine air to move over coastal mountains and spill into the interior valleys. If the air in the low pressure system is cold enough, the inversion that defines the top of the marine layer will dissipate, and so will the marine layer. Because of this, the maximum daily summer temperatures in the San Joaquin and Sacramento Valleys of California often undergo a roller coaster effect.

Bakersfield, CA Temperature Roller Coaster

Location of Bakersfield, CA

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Bakersfield California, located in the Southern San Joaquin Valley, is surrounded by mountains on three sides, with the only opening located to the northwest. The mountains block the moisture the valley would otherwise receive (annual rainfall for Bakersfield is only 6.49") as well as the cooling effect of the marine layer.

However, there are occasions when the marine layer deepens enough for the cooler air to pass over mountains and through passes, penetrating into the San Joaquin Valley. While the cooling effect is lower than on the coast due to the warm valley ground heating the marine air, the day-to-day changes in temperature can be dramatic.

After a few days, the marine layer again becomes suppressed below the mountain passes, and the sun gradually reheats the Southern San Joaquin Valley, only for it to be cooled again with the next marine layer surge.

The graph below shows the maximum temperature each day for Bakersfield, California in September 2004. The brown arrows indicate when marine air moved over the coastal mountains and reached the city.

The high temperature each day for Bakersfield, California, in September 2004

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How the marine layer affects temperatures.