CLIMATIC FACTORS : THE AIR 87 



Cooling of air masses is commonly caused by their expansion 

 when air rises in convection currents or when moving air is forced 

 to rise, as when it strikes a mountain slope. Cooling also occurs 

 cyclonically, for then masses of warm and cool air may meet, and, 

 depending upon which is least stable, warm air moves up over the 

 cool (a warm front) or the cold air underruns the warm (a cold 

 front). Of considerable local ecological significance is the contact 

 cooling resulting when relatively warm air moves over a cooler 

 surface or when cold air moves in over a body of warm water. 

 Under these conditions, fogs or clouds may form, which not only 

 may result in precipitation but may also modify the effects of 

 solar radiation. 



Fog.— Any minute particles of matter in the atmosphere with 

 hygroscopic properties may serve as condensation nuclei (there is 

 disagreement as to their necessity) about which droplets of water 

 form, the size of the droplets depending upon the speed of con- 

 densation. Contact cooling usually produces only small droplets, 

 which remain in the air and are visible as fog. Coastal fogs are of 

 this type when they are the result of prevailing winds coming off 

 the warm ocean and striking a cooler land mass. Such fogs are 

 usually dissipated as the day progresses, evaporating as the tem- 

 perature rises. Coastal fogs may also be caused by winds blowing 

 from areas of warm water across cool currents. In summer, along 

 the Pacific coast, warm air moves in from far offshore across the 

 cool California current flowing from the north. Fog forms over 

 the cold current and is blown inland, where it disappears if the 

 land is warm but persists at night when the land is cooler. Because 

 they affect light, temperature, and moisture conditions, fogs may 

 be of extreme importance in determining types of coastal vegeta- 

 tion and the agricultural possibilities of an area. The distribution 

 of coastal redwoods of our Pacific coast forms a striking example 

 of the effects of fog. In a region almost without summer rainfall, 

 the coastal redwood and several associated species are almost pre- 

 cisely limited to the humid fog belt along the coast. Fogs inland 

 are usually over low ground, swamps, or small bodies of water, 

 and are common in valleys where air movement is reduced and 

 radiation cooling is effective. 



Clouds.— Clouds differ from fog only in their position. Both are 



