that turns the prevailing northwest flow to a westerly and even southwesterly direction near the coastline, driving the marine layer and its clouds onshore and through any gaps in the Coast Ranges. The largest coastal gap is the Golden Gate at the entrance to San Francisco Bay which also communicates via the Bay with the Carquinez Strait and the Central Valley. As the city of San Francisco lies adjacent to the Golden Gate, it is often subject to the fog and low clouds blowing in on the marine layer. Even when the clouds are not present, the coolness of the marine layer exacerbated by the strong winds can chill the city even in mid-summer. Because of this, San Francisco is sometimes described as "naturally air conditioned". Under normal summertime conditions, a daily pattern of fog and low clouds occurs. Morning sun light heats the ground (cloud-penetrating visible light wavelengths transformed to infrared by the ground), which in turn heats the marine layer over the land areas. This creates convective turbulence within the marine layer as well as evaporation of any clouds within it. The marine layer clears back toward the coast, usually by noon. By mid-afternoon, inland areas have heated sufficiently to decrease the air pressure and increase the onshore flow. By late afternoon, the wind increases and begins to cool the onshore marine layer, allowing the fog and low clouds offshore to progress inland without evaporating. Cloudiness streams in over the Bay and through the various gaps. How far the clouds can penetrate inland depends on the depth of the marine layer and the strength of the cooling winds. As night falls and inland areas cool down, the winds usually decrease, but the fog and clouds remain wherever they have blown in until the following morning when the cycle repeats. Variations A land-sea temperature-pressure gradient is not always necessary to drive the marine layer and low clouds onshore into the Bay Area. Winds ahead of an approaching cold front or low pressure system can also push the marine layer onshore. Another pattern variation occurs in connection with heat spells that reach the coast from inland. Such heat waves typically occur when an area of high atmospheric pressure orients itself in such a way that the northerly to northeasterly gradient becomes dominant, driving the marine layer out to sea south and west of the California coast. These spells typically end