A marine layer is a dense mass of cool, moist air which accumulates over the surface of large bodies of water, especially oceans. It is caused by a temperature inversion, meaning that the air close to the ground is colder, rather than warmer, while warm air above the marine layer presses down on it, preventing it from dissipating. Marine layers are responsible for the fog which plagues many coastal communities, and they can also cause unusual phenomena, like campfire smoke which flattens out, rather than drifting upwards.

The formation of a marine layer can take place in a number of ways. Generally, air becomes saturated with moisture, causing it to become cool and dense, or it gets cold overnight and in cloudy conditions. Warmer air above will force the cold air back down, creating an inversion. A mass of warm air can also drift in over cooler air, trapping it so that the marine layer often gets colder and more dense as a result. If the air becomes saturated enough, the marine layer will turn foggy. These masses of cool air can also be quite large, extending well up into the atmosphere.

People in coastal communities are familiar with the marine layer in the form of an ominous pile of fog and clouds which hovers on the horizon, waiting for an opportunity to move onto shore. As conditions on shore cool, the marine layer drifts in, saturating the coastline with fog and sometimes light drizzle. Sometimes, inland communities can be extremely warm and sunny; this actually traps the marine layer even further, by creating a bubble of warm air enclosing the cooler, sluggish marine layer.

Many communities which experience marine layers also struggle with smog, particulate matter which pollutes the air. Smog can become trapped in the marine layer, and the same temperature inversion which causes the marine layer can also create a dense mass of smog. In some cases, smog may get so bad that citizens are warned that they should not go outdoors, as the air quality is extremely poor. Temperature inversions can also occur in valleys or deep depressions in the surface of the Earth, especially if they have large water features, which is why Mexico City struggles with smog, even though it is in the middle of Mexico.

Strong winds can break up a marine layer, often by pushing it onto shore so that the sun can dissipate it. Storms may also disperse marine layers, as will turbulence in the air column.