25th December 2005

Fog Stratus

Source: Excerpt from The Book " Weather "

Distribution: Worldwide, most common in inland areas. Height: 0 to 2000 feet deep. Cause: Lifting and erosion of a fog bank by solar heating. Associated Weather: Drizzle or light snow. Hazard Warning: May mask terrain and restrict visibility. Normally, a bank of fog forms during the night and begins to disperse as the Sun rises and warms the atmosphere. In certain conditions this can give rise to a bank of fog at a higher level. This phenomenon is known as fog, or low stratus. The Sun's rays first heat the ground near the edges of the fog, causing the perimeter to dissipate. Some of the heat also penetrates the deck, warming the ground underneath. The heat from the ground then begins to evaporate the fog at low level. Thus, the fog erodes from the edges toward the center and from the underside up, resulting in a layer of fog some distance off the ground. To someone standing within the fog, this process is usually signaled by a gradual increase in visibility at ground level. However, if the deck remains intact as it rises, the Sun may stay hidden for some time. Because fog stratus forms in still conditions, there is seldom any wind at ground level and the stratus normally lifts more or less vertically as it erodes. On some occasions, however, light winds may develop soon after the bank of stratus has formed, and gently blow the fog across the ground as it is dispersing. This will tend to speed up the clearing process. Generally, fog stratus will have cleared by mid to late morning, although exceptionally thick layers may take longer to disperse. Sometimes, an increase in middle or high level cloud can inhibit the heating process at ground level and result in the fog stratus remaining intact for most of the day. This in turn will keep temperatures low at ground level. A thick layer of fog stratus may produce light drizzle, or snow in cold temperatures. However, because the fog tends to get thinner as it rises, any precipitation is generally short-lived. Extensive areas of fog stratus may obscure large areas of terrain, creating a significant aviation hazard. This can cause problems for the motorist as well, particularly in mountain areas. Valley roads may be clear, but as the motorist ascends he or she may suddenly encounter thick fog, before emerging once again into bright sunshine above the layer of fog stratus.

---

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