TEMPERATURE OF THE AIR 247 



4. The great bend in the January isotherm of 30 in the North 

 Atlantic is due to a warm current of ocean water flowing northeast- 

 ward, in the direction of the pronounced loop of the isotherm. Ocean 

 currents are therefore a fourth cause of the irregularities of isotherms. 

 The amount of heat carried northward by the ocean currents of 

 the Atlantic and Pacific is very large. It has been estimated that 

 the temperature of England and Norway is raised several degrees 

 by the warm poleward movement of waters in the North Atlantic. 

 The temperature of the land is warmed by this water, because the 

 air over the warm ocean water is warmed and then blown over the 

 land. 



The milder climate of northwestern Europe, as compared with 

 northeastern North America, is not due wholly to the northward 

 movement of warm water. Even without such movement, the 

 climate of northwestern Europe would be more temperate than 

 that of northeastern North America in the same latitudes, because 

 the ocean, from which the winds of winter blow to that part of 

 Europe, is warmer than the land whence the winds blow to the 

 lands of the same latitude on the west side of the Atlantic. 



There are some other less important causes of irregularities in 

 the isotherms. Thus a basin region, shut in by mountains, gets 

 hotter in summer than a region not so surrounded. Again, there 

 is less evaporation from a dry surface than from a moist one, and 

 since evaporation cools the surface, a dry surface will be warmer 

 than a moist one if other conditions are the same. The color of the 

 soil, the presence or absence of vegetation, etc., also affect the 

 absorption and radiation of heat. The high temperature (90 and 

 above) in the southwestern part of the United States in July is 

 accounted for partly by the fact that the region is somewhat shut in 

 by mountains. The dryness of the soil and of the air above it also 

 tend to raise its temperature. 



Altitude affects temperature, as already explained, but isother- 

 mal charts show no relation between isothermal lines and surface 

 relief. The reason is that isothermal lines are represented on maps 

 as if they were at sea-level. This is done by making allowance for 

 altitude at the average rate of 1 F. for about 330 feet. Thus if 

 the temperature of a place at an altitude of 3,300 feet is 60, it is 



