268 PHYSIOGRAPHY 



ture which is largely determined by latitude, which influences the 

 pressure, and gives the isobars an east-west course? 



If temperature controls the position of isobars, they should he 

 lowest at the equator, where it is wannest, and highest at the poles, 

 where it is coldest. Fig. 222 shows that this is not the case, and 

 it shows that pressures are distributed in apparent disregard of 

 temperature. The isobars are highest neither where it is coldest 

 nor where it is warmest. It is clear, therefore, that neither latitude 

 nor temperature, nor both together, control entirely the position of 

 isobars. 



Relation of isobars to land and water. The isobars are much 

 more regular in the southern hemisphere, where there is much water, 

 than in the northern hemisphere, where there is less. This suggests 

 that the distribution of land and water influences the position of 

 isobars. The land is warmer than the sea in the same latitude in 

 summer, and cooler in winter; and anything which influences tem- 

 perature should influence pressure also. 



Isobars and temperature. The isobaric map for January (Fig. 

 227) shows that the high-pressure (more than 30 inches) belt is very 

 wide in the northern hemisphere (winter), especially on the land, 

 which at this season is cooler than the sea. This supports the in- 

 ference that high pressure goes with low temperature. In the south- 

 ern hemisphere, January is a summer month, and the land is warmer 

 than the sea. If high temperature causes low pressure, the pressure 

 in the southern hemisphere at this time should be less than that 

 in the northern, and it should be lower on the land than on the sea. 

 The map shows that both these things are true. This chart, there- 

 fore, seems to show that high temperature reduces pressure. 



A study of the isobaric chart for July (Fig. 228) leads to the 

 same conclusion. At that time of year, the pressure in the south- 

 ern hemisphere (winter) should be higher, on the average, than in 

 January (Fig. 227). Especially should it be higher on land, as the 

 map shows it to be. In the northern hemisphere (summer), on the 

 other hand, the pressure should be less than it was in January, and 

 especially should it be less on land, which is much warmer than 

 it was in winter. Fig. 228 shows both these things to be true. 

 We have confidence, therefore, in the conclusion that high tern- 



