# Marine layer

A marine layer is an air mass that develops over the surface of a large body of water, such as an ocean or large lake, in the presence of a temperature inversion. The inversion itself is usually initiated by the cooling effect caused when cold water on the surface of the ocean interacts with a comparatively warm air mass.

## Elements

A marine layer can come in a number of different forms depending on the atmospheric conditions. It may manifest itself as merely a cool, humid air mass without any cloud cover, or it may be accompanied by clouds. In many cases, marine layers can consist of dense fog. Often associated with marine layers are stratus clouds, which are lumpy, often uniform clouds that form at low elevations (less than 3000 feet). Since marine layers are pushed ashore by westward winds, wind will almost always be present in one. Ultimately, the marine layer is a medium within which clouds may form under the right conditions; it is not the layers of clouds themselves.

## Formation

As air cools in a temperature inversion, the surface air becomes denser than the warmer air above it, and thus becomes trapped below it. The lower layer will also gradually increase its humidity by evaporation of the surface water, as well as by the effect of the cooling itself, since cooler air holds more moisture.
Clouds and potentially fog can form within a marine layer when the water-saturated air is cooled and reaches a humidity of 100%, where it will then condense and turn into water droplets. Stratus and stratocumulus can form at the top of a marine layer in the presence of these conditions. A marine layer can often be dispersed by sufficiently strong winds, which essentially blow it away. The sun can also evaporate the marine layer, as it warms air and land which decreases the relative humidity and causes marine layer clouds to evaporate.

## Notable examples

In the case of coastal California, the offshore marine layer is enabled by the cold relative sea surface temperatures of the Pacific Ocean. California receives this unusually cold water through a process called the California Current, where cool polar water is transported from the Gulf of Alaska to the California coast. This process creates anomalously cool water temperatures given the latitude of California, and causes a strong temperature inversion. Surface waters in coastal California are kept cold through a process called upwelling, which involves cooler, deeper waters rising to the top of a water column.
Occasionally the marine layer becomes particularly deep, and the clouds on land can persist all day. This can happen at any time of the year, inspiring colloquialisms such as "May Gray" or "June Gloom". An approaching frontal system or trough can also drive the marine layer onshore.
Though California is a notable example of the marine layer, these atmospheric phenomena exist in several other places around the world. Marine layers often form on the west coasts of continents, where upwelling usually occurs, and where there is a relatively warm air mass moving over a cool body of water. They can also form on the coasts of cool lakes, such as the Great Lakes in the United States.