THE ATMOSPHERE, WEATHER, AND CLIMATE 



13 



whether they beheved the sun was closer to earth in 

 the winter or in the summer, some might assume that 

 "summer" was the answer. However, the exact op- 

 posite is true. The essentially perpendicular rays 

 produce the summer when the earth is farther away 

 from the sun than it is in the winter. 



Clean, dry air provides less obstruction to sunlight 

 than does dustier, moister air. The former has fewer 

 particles to act as obstructions to the rays, hence the 

 maximum radiation reaches the lower atmosphere 

 and the earth's surface. However, the conditions for 

 maximum daytime heating are also those of maxi- 

 mum nighttime cooling. The night is cooler because 

 a minimum amount of water vapor and dust provide 

 a minimum amount of heat insulation for the surface. 

 This is why areas characterized by clean, dry air 

 (e.g., deserts) are noted for their relatively great 

 fluctuation in temperature. 



The land is subject to faster and greater tempera- 

 ture changes than is water. This difiference stems 

 from a particular property of water — it retains heat 

 longer and acquires heat more slowly than most other 

 substances (see Figure 3.1, p. 28). This characteris- 

 tic helps to explain why most of the world's ocean 

 area has annual temperature extremes no greater 

 than 8° F. and why atmospheric water vapor is the 

 most important factor in preventing great air tem- 

 perature extremes. In daytime it inhibits rapid in- 

 creases and at night, rapid losses. 



EFFECTS OF TEMPERATURE VARIATIONS 



In the lowest zone of the atmosphere (troposphere) 

 there are changes in temperature at diflferent alti- 

 tudes. Because the greatest air density is at the sur- 

 face of the earth and hence most particles are there, 

 the highest and most uniform air temperature is 

 also at the surface. However, there are modifica- 

 tions within this pattern. If a localized surface area 

 is warmed, this air expands and its pressure de- 

 creases. Any pressure reduction also means lighter 

 air so it is forced upward by the pressure of the denser 

 air around it. As it rises, it reaches surrounding air 

 of still lower pressure. Therefore, rising air expands, 

 and with expansion, it cools. Conversely, if higher, 

 heavier air descends, the surrounding air progres- 

 sively compresses the descending air, increasing its 

 air pressure and temperature. For these reasons, at- 

 mospheric temperature is frequently related to atmos- 

 pheric pressure. 



Also associated with temperature, and hence the 

 upward and downward movements of air masses, is 

 the capacity of air to hold moisture. As air rises, 

 cooling reduces its capacity to hold moisture, so the 

 water vapor condenses into water droplets, thus form- 

 ing clouds. The point at which condensation takes 

 place is just beyond the maximum amount of water 

 vapor the air can hold at a given time and is termed 

 the saturation or dew point. (Dew is produced on the 

 ground through the same process.) On the other 

 hand, as air descends, warming increases its mois- 

 ture-carrying capacity (Figure 2.3). 



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Figure 2.3 Temperature, pressure, volume, and condensotion relotion- 

 sbips in ascending and descending air. As air rises its temperature and 

 pressure decrease but its volume increases. Related to decreasing tem- 

 perature is the onset of v/ater vapor condensation, the dew point, which 

 causes air to rise even more and furthers the above relationships. Pro- 

 gressive condensation causes clouds, then precipitation. As air descends, 

 the opposite conditions prevail. 



Water condensation adds further complexity to air- 

 temperature relations. Any water condensation in- 

 volves a so-caUed heat of condensation. This heat is pro- 

 duced by the process of condensation and is released 

 into the surrounding air. For this reason, the phe- 

 nomenon in which air rises and cools results in heat 



