The marine layer is the influence of the air mass that develops over the open water than moves inland. The marine layer is of great significance to coastal weather such as on the west coast of North America. The marine layer brings in clouds, cooler temperatures and light precipitation. The cooler temperatures are often influenced by a cold ocean current. The evaporation of moisture into the cool air allows saturation to occur fairly easily. Thus the marine layer is often at or near saturation and because of this clouds and fog are often present. The marine layer is often shallow and this has very important weather forecast implications.

The marine layer is many times frustrating for weather forecasters. This is especially true for places where the marine layer is adjacent to a much warmer continental air mass. The difference in weather with and without the marine layer present can be vastly different. The challenge a forecaster faces is determining when the marine layer will be present. The marine layer has its greatest influence overnight and in the morning. This is because solar radiation is less likely to mix it out at this time of day. The marine layer can mix out during the day since it is shallow. Much drier and warmer continental air when mixing with marine layer air will cause the moisture of the clouds and fog to evaporate away and this allows further solar warming from the sun. A forecast challenge is determining when the marine layer will mix out. If the marine layer ends up not mixing out when it is expected to then it can lead to vastly different weather in the afternoon. The marine layer also tends to hang on longer near the coast than further inland. Thus, the forecast area can experience vastly different weather.

The marine layer is helped by a wind flow from the ocean, a synoptic pattern that allows for the cool/saturated air mass to build and a cold ocean current. The diagram below depicts the marine layer over the ocean and over land near the coast. In the afternoon the marine layer can mix out with dry and warm continental air leading to sunny weather.