National Weather Service United States Department of Commerce

The Marine Layer

Somewhat similar to the sea breeze, the marine layer also represents a difference between a cool, moist air mass and a warmer air mass. Unlike the sea breeze, which reforms almost every day along the east coast in Summer, the marine layer can persist for days and weeks along the west coasts of continents. This is particularly seen along the coasts of Central and Southern California.

In Summer, it is not uncommon for someone to drive across the San Francisco's Golden Gate bridge in fog with the temperature in the upper 50s to lower 60s. Yet only to go to the top of Mount Tamalpais, just a few miles north of the bridge (elevation 2,571 feet/ 784 meters), the skies will be clear skies with a temperature in the 80s or even the lower 90s, all because of rising above the cool marine layer.

Golden Gate bridge towers sticking up above the marine layer.

The reason is the water along the west coast of the United States comes from the Gulf of Alaska and is much cooler than at the same latitude along the east coast where the Gulf Stream brings warmer tropical water north. The surface temperature of the water off the California coast can be as much as 30°F (17°C), or more, lower than at the same latitude on the east coast. The colder water means the air in contact with the water is colder and therefore denser.

Air temperature normally decreases width height. 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. Because of its persistence in early Summer, the people in Southern California it is often refer to it as the "May Gray" or "June Gloom". This can occur near any large body of water such as the Great Lakes region when the water temperature is significantly colder than the air moving over it.

The depth of the marine layer depends upon the large-scale weather patterns that pass high overhead. Sinking air, under 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.

The strength of the sinking air impacts the depth of the marine layer. If the downward force of air is very strong, the marine layer is very shallow with low clouds and foggy weather confined to the beaches and very warm, sunny conditions beginning just a mile or so inland.

Any decrease in downward forcing allows the marine layer to deepen and move farther inland. Near the beach, the fog lifts into a low cloud layer. The leading edge of the marine layer extends farther inland pushing the fog inland.

Strong downward motion from upper level high pressure squashes the marine layer and keeps is confined to the coast. Inland temperatures are high.
A weakening of the downward motion aloft allows for the marine layer to move further inland and a little lower inland temperatures.
Slight upward motion in the upper atmosphere allows the marine layer to move well inland but tall coastal mountains inhibit further penetration. Inland valley temperatures will continue to decrease slightly due to the overall upward motion of air.
Strong lifting aloft considerably deepens the marine layer allowing cooled marine air to spill over coastal mountains into the inland valleys.

Further lifting of the marine layer will allow cooler marine air move over the coastal mountains and spill into the interior valleys. Because of this, often the maximum daily temperatures in the San Joaquin and Sacramento Valleys of California will undergo a roller coaster effect in Summer.

Bakersfield, CA Temperature Roller Coaster

Location of Bakersfield, CA

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 are responsible for the low annual rainfall amounts each year by blocking the moisture it would otherwise receive (annual rainfall for Bakersfield is 6.49"). These mountains also normally block 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 will not be as much as the coastal sites (the warmth of the ground modifies the marine air), the changes in temperature from day to day can be dramatic.

Then, after a few days, the marine layer becomes suppressed again below the mountain passes and the sun gradually reheats the Southern San Joaquin Valley only 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