Fog

Advection fog forms when warm, moist air moves horizontally over a relatively cooler surface. During such contact, the layer of air near the surface may cool to below its dew point to form advection fog. Because advection fog can form at any time, it can be very persistent. It is common along coastlines where moist air moves from over a water surface to a cooler coastal land mass. Advectional fog can also occur if an already cool air mass moves over a still colder surface (e.g. snow), so that even the reduced levels of moisture in the cold air can condense into fog as the surface continues to cool the air mass. Advection-radiation fog forms when warm, moist air moves over a cold surface that is cold as a result of radiation cooling. When warm, humid air moves over cold water, a sea fog may form.

Dense fog engulfing the Golden Gate Bridge (San Francisco, CA). CORBIS/Kevin Schafer. Reproduced by permission.

Upslope fog forms in higher areas, where a moist air mass is forced to move up along a mountain incline. As the air mass moves up the slope, it is cooled below the dew point to produce fog. Upslope fog formation generally requires a stronger wind along with warm and humid conditions at the surface. Unlike radiation fog, this type of fog dissipates as wind dissipates, and it can form more easily under cloudy conditions. Upslope fog is usually dense, and often extends to high altitudes.