264 PHYSIOGRAPHY 



In the southern hemisphere, the isobar of 30 inches encloses 

 a belt extending almost around the earth, being interrupted only 

 in the vicinity of Australia. Every point within the area enclosed 

 by this isobar has an average atmospheric pressure of more than 30 

 inches. Every point within the isobar of 30.10 inches has an aver- 

 age annual pressure of more than 30.10 inches, while every point 

 between the isobars of 30 and 30.10 has an average annual pressure 

 of more than 30 and less than 30.10 inches, etc. Between the two 

 adjacent isobars of 29.90 in the equatorial part of the Atlantic, 

 the pressure is less than 29.90, but not so low as 29.80. If the pres- 

 sure sank to the latter figure, there would have been isobars of 

 29.80 inches. 



It will be remembered that the temperatures shown on land or 

 an isothermal chart are not those actually observed, but that allow- 

 ance is made for altitude above sea-level. In the same way, the 

 pressures shown on land on an isobaric chart are not those actually 

 observed. They are the pressures which would exist if there were no 

 elevation above sea-level. The allowance which must be made for 

 elevation above sea-level varies with the temperature and the 

 pressure. When the temperature is 70 F. and the pressure 30 

 inches, 95 feet of elevation diminishes the pressure 0.1 of an inch. 

 If the pressure 95 feet above sea-level is 30 inches, it would be put 

 down on the isobaric chart 30.1. If the temperature were lower, 

 0.1 of an inch would be added for a slightly lesser height, since 

 colder air is heavier. 



Isobaric surfaces. An isobaric surface connects places having 

 the same pressure, that is, the same weight of air above. If, for 

 example, one place at sea-level had a pressure of 30 inches, and 

 another a pressure of 30.10 inches, the isobaric surface of 30 inches 

 would lie above sea-level at the place where the pressure is 30.10 

 at sea-level. If the pressure at sea-level at another place were 

 29.90 inches, the isobaric surface of 30 inches would be below sea- 

 level there. An isobaric surface, therefore, has slopes. 



Fig. 223 shows a series of isobars, with the pressure least at the 

 center, and Fig. 224 shows the slope of the isobaric surfaces in the 

 same place. Fig. 225 shows another series of isobaric lines, with 



