WEATHER MAPS 



287 



a temperature intermediate between 40 and 50. The isotherm 

 of 30 is still more irregular. 



The isotherms of Fig. 239 show two distinct features: (1) they 

 have little relation to parallels, and (2) the isotherms bend north- 

 ward where the pressure is low, and southward where the pressure 

 is high. This last feature is shown in most of the weather maps 

 which follow; but on many of them the isotherms follow the 

 parallels more closely than in Fig. 239. 



The temperature, the pressure, the winds, the cloudiness, the 

 rain, etc., are the elements of the weather. All these things being 

 shown on the above map, it is appropriately called a weather map. 



Fig. 242. Diagram illustrating the general position of air currents in a 

 cyclone of intermediate latitudes, and the fact that the upper air moves 

 mostly toward the east, in the direction of the prevailing winds. (U. S. 

 Weather Bureau.) 



The lows and highs are sometimes much more pronounced than 

 those shown in Fig. 239. In Fig. 243 the low is more pronounced, 

 vhe pressure ranging from 29 at the center to 30.1 in the east, and 

 to 30.5 in the west. So great a range of pressure within the United 

 States is not common. The isobars are closer together in this 

 figure than in Fig. 239, and therefore indicate stronger winds. 

 Cloudy skies prevail in the southeastern part of the cyclone. 



Highs as well as lows may have great area. Fig. 244 shows a 

 high, or anticyclone, more than 2,000 miles across, with a great 

 range of pressure. The isotherms of this chart, like those of the 

 preceding, stand in very definite relations to the isobars. Denver, 

 in the anticyclone, is about 30 colder than the southern part of 

 Maine, which is 3 farther north, but in a cyclone. 



Around cyclones precipitation takes place in many cases, 

 while around anticyclones there is, as a rule, an absence of pre- 

 cipitation. The chief reason for rainfall or snowfall about a low 



