Page 234 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

198 part II The Water, Weather, and Climate Systems
▲Figure 7.21 advection fog. San Francisco’s Golden Gate Bridge is shrouded by an invading advection fog characteristic of summer conditions along a western coast. [Brad Perks lightscapes/Alamy.]
directly above the surface becomes chilled to the dew point and fog develops. Off all subtropical west coasts in the world, summer fog forms in the manner just de­ scribed (Figure 7.21).
One type of advection fog forms when moist air flows to higher elevations along a hill or mountain. This upslope lifting leads to adiabatic cooling by expansion as the air rises. The resulting upslope fog forms a stratus cloud at the condensation level of saturation. Along the Appalachians and the eastern slopes of the Rockies, such fog is common in winter and spring.
Another advection fog associated with topography is valley fog. Because cool air is denser than warm air, it settles in low­lying areas, producing a fog in the chilled, saturated layer near the ground in the valley (Figure 7.22).
Evaporation Fog Another type of fog that is related to both advection and evaporation forms when cold air lies over the warm water of a lake, an ocean surface, or even a swimming pool. This wispy evaporation fog, or steam fog, may form as water molecules evaporate from the water surface into the cold overlying air, effectively humidifying the air to saturation, followed by conden­ sation to form fog (Figure 7.23). When evaporation fog happens at sea, it is a shipping hazard called sea smoke.
The prevalence of fog throughout Canada and the United States is shown in Figure 7.24. Every year the media carry stories of multicar pileups on stretches of highway where vehicles drive at high speed in foggy con­ ditions. These crash scenes can involve dozens of cars and trucks. Fog is a hazard to drivers, pilots, sailors, pe­ destrians, and cyclists, even though its conditions of for­ mation are quite predictable. The distribution of regional fog occurrence should be a planning consideration for any proposed airport, harbour facility, or highway.
Fog is an important moisture source for many or­ ganisms. Humans are increasingly using fog as a water resource in some regions. Throughout history, people have harvested water from fog. (See Geosystems Now at the beginning of this chapter for more on this topic.)
▲Figure 7.22 Valley fog. Cold air settles in the Saar River valley, Saarland, Germany, chilling the air to the dew point and forming a valley fog. [Hans-Peter Merten/Getty images.]
▲Figure 7.23 evaporation fog. evaporation fog, or sea smoke, de- velops on a very cold morning on Halifax Harbour. later that morning as air temperatures rose, what do you think happened to the evapora- tion fog? [Andrew Vaughan/Canadian Press images.]