Fog is a type of stratus cloud. Unlike other clouds, fog forms at ground level when the air on the ground cools, and water vapor condenses into suspended water droplets. This typically occurs in humid weather.
Two of the more common types of fog include radiation and advection fog. Radiation or ground fog develops overnight as the earth cools, and burns off with the morning sun. Radiation fog causes low visibility and is often responsible for hazardous driving conditions. Advection fog looks similar to radiation fog, but advection fog occurs as warm and moist air moves over a cold surface. This type of fog typically appears on the Pacific coast, when warm air meets the cold California Current.