The marine layer is a cool and humid layer produced by cooling from a cool ocean current and evaporation from the ocean that humidifies the air. This layer of air can stay off shore or move well inland depending on the wind direction. When the marine layer is in place, it is often accompanied by cloudy and/or foggy conditions. This causes the weather to be cooler since sunlight is not effective in warming the surface. What makes the marine layer a weather changer is that this layer moving in or moving out causes a dramatic change in the weather.

One location that is influenced by the marine layer is the West Coast of the U.S. For the West Coast, the marine layer can cause some of the biggest shifts in weather. When the marine layer is in place, the weather can be cool, cloudy and damp. When the marine layer retreats or mixes out, then the weather can be hot, clear and dry. The marine layer can cause havoc with weather forecasting since an incorrect forecast on marine layer timing can cause the forecasted high temperature to be way off. For locations near the coast, the marine layer will often be in place in the morning and then by afternoon the sunís influence will be enough to mix out the marine layer. If the marine layer is strong enough though then it will not mix out and the entire day can end up being cool, cloudy and damp. For example, this layer mixing out or not can mean the difference between a high of 75 F and a high of 90 F.