ATMOSPHERIC PRESSURE 263 



of the barometer. A special form of barometer, the aneroid barome- 

 ter, has been devised for this purpose. 



Air pressures unequal. The pressure of the atmosphere varies 

 from point to point, and from time to time at 

 the same point. Some of the reasons are as follows : 



1. The temperature of the surface on which 

 the air rests is unequal, and increase of temperature 

 makes the air lighter. As the temperature varies, 

 the pressure varies. 



2. Water vapor in the air makes the air lighter 

 because it crowds out some of the oxygen, nitro- 

 gen, etc., which weigh more than the vapor. The 

 amount of moisture in the air is greater in warm 

 regions (but not in hot deserts) than in cold ones, 

 and greater over moist surfaces than over dry ones. 

 Since the amount of moisture in the air varies from 

 time to time, the pressure is constantly changing. 



If temperature and moisture were the only 

 factors controlling air pressure, the pressure would 

 be least in low latitudes where it is warmest, and 

 where there is abundant moisture. Since atmos- Fig. 221. Dia- 



pheric pressure is not least in low latitudes, even g ram * illus- 

 trate the prm- 

 where it is moist, we conclude that temperature ciple of the 



and moisture are not the only things which affect it. barometer. The 



air at A main- 

 Representatwn of Pressure on Maps and Charts tains the mer- 



ij cur y at B in the 



Isobars. Lines may be drawn on the sur- tube when there 



face of the earth connecting points where the ! s 7 10 a _ ir m * hc 



. . tube above B. 



atmosphenc pressure is the same. Such lines 



are isobars. A map showing lines of equal pressure is known 

 as an isobaric map or chart. An isobaric chart of the year, 

 that is, an annual isobaric chart, shows isobars connecting points 

 having the same average pressure throughout the year. There may 

 be isobaric charts for a season, for a month, or for any shorter 

 period. The daily weather maps are daily isobaric charts. Fig. 

 222 represents an isobaric chart for the year. The figures on the 

 lines indicate the average pressure for the year in inches. 



