Fog
Fogs are clouds located at or near the ground. When humid air near the ground cools below
its dew point, fog is formed. Fogs develop differently from the way clouds form. There are
several types of fog, each of which forms in a different way.
Radiation fogsform at night when skies are clear and the relative humidity is high. As
the ground cools, the bottom layer of air cools also. Eventually the air temperature may
be lowered below its dew point. If there is a light breeze, the fog will be carried upward.
Radiation fog can grow to 30 meters (100 feet) thick. One to three hours after sunrise,
radiation fog burns off as the ground warms. The Central Valley of California frequently
experiences radiation fog, which is called tule fog in this area. Tule fog can be so thick that
drivers cannot see the car in front of them and their headlights just reflect back off the sheet
of water droplets.
SanFrancisco,Californiaisfamousforitssummertimeadvectionfog(Figure16.9). Warm,
moist air from over the Pacific Ocean blows over the cold California current just offshore.
This cools the eastward moving air below its dew point and thereby creates fog. Advection
fog is brought onshore by sea breezes. If the fog is accompanied by light wind, a thicker
layer of air cools and the fog can grow to be up to 600 m (2,000 feet) thick.
Figure 16.9: Advection fog fills the gap where the Golden Gate Bridge spans the San Fran-
cisco Bay inlet. ( 16 )
Steam fog appears in autumn or early winter and can make a pond or lake appear to
be steaming. The “steam” forms when cool air moves over a lake that still holds some of
its summer heat. Water evaporates from the lake surface and condenses as it cools in the
overlying air. Steam fog is rarely very thick.
When warm humid air travels up a hillside and cools below its dew point it creates an
upslope fog(Figure16.10).