ATMOSPHERIC PRESSURE 271 



perature reduces the pressure, while low temperature increases it. 

 The charts furnish other evidences in support of the same con- 

 clusions. 



Isobars and humidity. We have seen (p. 225) that water vapor 

 makes the air lighter. But the isobars are not lowest over the 

 oceans in warm latitudes, where the air contains on the average 

 most moisture. We conclude, therefore, that the amount of moisture 

 in the air is not the chief factor controlling the isobars. 



Inequalities of temperature and moisture in the air are the only 

 factors thus far studied which might affect the isobars; and since 

 they do not explain the most striking feature in the distribution 

 of atmospheric pressure, namely the high pressures in low latitudes, 

 we conclude that something besides temperature and moisture 

 must be involved in their explanation. 



The high-pressure belts. The explanation of the high pressure 

 in low latitudes, rather than in high, and the explanation of the 

 highest pressures just outside the tropics, is not found on the iso- 

 baric charts. These larger features of pressure-distribution are 

 to be explained by the general circulation of the atmosphere tinder 

 the influence of rotation. The details of this explanation are here 

 omitted. 



Temporary and local variations of pressure. There are many 

 variations of pressure not shown on seasonal or even on monthly 

 isobaric charts, though they appear on daily weather maps. These 

 will be studied in a later chapter. 



