24 



METEOROLOGY AND CLIMATOLOGY: 



characteristics of the winds of our planet. For ex- 

 ample, in the winter ocean storms associated with the 

 very low pressure of subpolar areas occur in the mid- 

 dle and high latitudes. In the summer, ocean midlati- 

 tudes are dominated by the very high pressure of sub- 

 tropical areas and the weather is usually mild. Also, 

 in the summer, tropical hurricanes may form and 

 move into temperate seas. As a consequence of all 

 these ocean conditions, the west coasts of continents 

 have rainy winters and dry summers. However, 

 topographic features reduce the amount of precipita- 

 tion inland (Figure 2.11). In California during 

 the winter the outer Coast Ranges cause coastal pre- 

 cipitation by forcing moisture-laden air from the 

 Pacific Ocean to rise and cool. Then air travels 

 downward into valleys between the outer and inner 

 Coast Ranges, warms, and is less likely to lose much 

 water vapor. Such interior, more arid valleys are 

 said to be in a rain shadow. From these valleys air 

 rises over the inner Coast Ranges, but usually not 

 enough to cause marked precipitation. The air then 

 drops into the severe rain shadow of the Great Valley, 

 a true semidesert, or steppe. However, the marked 

 rise and cooling of air over the Sierras once again 

 causes significant precipitation. Finally, the 2-mile 

 drop of air (a foehn wind) into the Great Basin again 

 produces great aridity. 



West Coast winter climate is further modified by 

 ocean conditions. The prevailing westerlies bring the 

 moist air that is released in association with the pat- 

 tern of coastal and interior mountains and plains. 

 However, the westerlies are relatively warmer and 

 moister in the north than in the south. Hence, there 

 is a progressive reduction in precipitation from the 

 Canadian to the Mexican borders. This moisture dif- 



ferential is a consequence of the fact that the waters 

 of the southern-moving California Current are colder 

 than their surroundings in the south as compared 

 with conditions in the north. Because the water is 

 relatively colder, the southern air obtains less water 

 vapor from the sea, so less moisture is available for 

 precipitation upon the land. 



CLIMATIC FACTORS AND DESERTS 



The operation of climatic factors (pressure, lati- 

 tude, ocean currents, topography, and continental 

 distribution) is seen readily in the formation of 

 deserts. Pressure is difficult to separate from the 

 other factors. Perhaps the purest consequence of 

 pressure alone is related to latitude. Recall that on a 

 world basis, four "Highs" and three "Lows" exist. 

 The "Highs" are the two polars and two subtropicals 

 (the latter near latitude 30°, above the Tropic of 

 Cancer and below the Tropic of Capricorn). The 

 three "Lows" are the equatorial and two subpolars 

 (just beyond latitude 60° in the vicinity of the Arctic 

 and Antarctic circles). Also, recall that air rises in 

 "Lows" and often leads to precipitation and that air 

 descends in "Highs" and often leads to clear skies. 

 Therefore, in the vicinity of the world "Highs" 

 deserts are likely to be found. 



Cold ocean currents or upwelling of deep, cold sea 

 water may result in dry winds over adjacent land 

 masses. Because ocean currents are related to air 

 currents, cold waters generally intensify the effects 

 of pressure and latitude. 



Topography can influence climate through a va- 

 riety of its features. Distance alone is sufficient to 

 remove most moisture from winds. However, moun- 



PACIFIC 



Figure 2.1 1 Climate and geography. The general consequences of oceanic air's rising and descend- 

 ing over the variable topography across centrol California. 



