25th December 2005

Advection Fog

Source: Excerpt from The Book " Weather "

Distribution: Worldwide, most common at sea and in coastal areas. Height: 0 to 1000 feet deep. Cause: Moist air moving into cold environment, or cold air moving into moist environment. Associated Weather: Drizzle or light snow. Hazard Warning: Restricted visibility. Advection fog often looks like Radiation Fog and is also the result of condensation. However, the condensation is caused not by a reduction in ground temperature, but by moist air drifting into a cold environment (or cold air moving into a moist environment). This means that advection fog can sometimes be distinguished from the normally stationary radiation fog by its horizontal motion. Since radiation fog almost always forms at night, any fog forming during the day is likely to be advection fog. Sea fogs are always advection fogs, because the oceans don't radiate heat in the same way as land and so never cool sufficiently to produce radiation fog. Fog forms at sea when warm air associated with a warm current drifts over a cold current and condensation takes place. Sometimes such fogs are drawn inland by low pressure, as often occurs on the Pacific coast of North America. Advection fog may also form when moist maritime air drifts over a cold inland area. This usually happens at night when the land temperature drops as a result of radiational cooling. Another common form of advection fog is valley fog. In this case, air that has cooled (and thus become denser) during the night drains into a valley from surrounding hillsides. Condensation then takes place, and the valley fills with fog.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.