Summers on the coast are cool to mild with frequent fog. Yet just 10 or 20 miles inland one can find abundant sunshine and warmth. Thus coastal residents often head eastward in the summer to escape the gloomy cold summer fog. Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snow pack. It’s most common at sea when tropical air encounters cooler waters, or in areas of upwelling, such as along the California coast. The advection of fog along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coastward, an occurrence most typical in the spring or late fall. During the summer months, a low pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer monsoon, produces a south to southeasterly flow which can drive the offshore marine layer up the coastline, a phenomenon known as a "southerly surge", typically following a coastal heat spell.