25th December 2005

Radiation Fog

Source: Excerpt from The Book " Weather "

Distribution: Worldwide. Height: 0 to 1000 feet deep. Cause: Cooling of ground causing condensation in air layers near surface. Associated Weather: Drizzle or light snow. Hazard Warning: Restricted visibility. Fog is really cloud that forms near the ground, and like cloud, fog forms as a result of condensation. Probably the most common form of fog is radiation fog, so named because it is produced by radiational cooling of the ground. This happens at night, when heat absorbed by the Earth's surface during the day is radiated back into space. The highest degree of radiational cooling occurs on clear nights, when there are no clouds to reradiate the heat back to Earth. Radiational cooling produces condensation in the air layers immediately above the ground. If only a thin layer of moist air is present, Dew will form; if a thicker layer is present, radiation fog (and dew) will form. Radiation fog varies in depth from only 3 feet to about 1,000 feet. As it is always found at ground level, the most obvious effect of this type of fog is a reduction in visibility, which may drop to as low as 10 feet in thick fog. If visibility is between 0.5 and 1.25 miles (1 and 2 km), the fog is known as mist. If any smoke is present, it may combine with the fog to produce smog. Fog usually disperses soon after sunrise, as the Sun's rays gradually warm the ground. Because clear skies are required for radiation fog to occur, a fine day normally follows. In some cases, however, middle-level cloud may slide over the fog early in the day, inhibiting the clearing of the fog by the Sun. Dense radiation fogs have caused many aviation and motoring accidents over the years. Even today, despite sophisticated onboard navigation equipment, aircraft landings in thick fog are not normally permitted.

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