276 



PHYSIOGRAPHY 



north to south, and the direction of the wind (Fig. 233) is in har- 

 mony with the planetary system (Fig. 231); but in Fig. 234 the 

 isobaric gradient is to the northward, because the land is warmer 

 than the sea, and the winds in the lower part of the air blow as 

 show in Fig. 235. That is, the planetary (northeast) wind 

 is overcome during the hot season by the winds which result from 

 the seasonal change of temperature. At the same season, the low 

 pressure north of India, developed by the heat of summer, coun- 



Fig. 236. Fig. 237. 



Fig. 236. Isotherms of India for January. (Buchan.) 

 Fig. 237. Isotherms of India for August. (Buchan.) 



teracts the high pressure common in this latitude, and the pre- 

 vailing trade wind is displaced by seasonal winds. Figs. 236 and 

 237 show the isotherms for the same region at the corresponding 

 seasons, and make clear the relation between pressure and tem- 

 perature. 



Spain, in the zone of westerly winds, affords another excellent 

 example of the monsoon winds. The general principle of the 

 monsoon makes itself felt about the Great Lakes. 



Besides the planetary winds, the seasonal winds (monsoons) 

 and minor periodic winds (land and sea breezes, etc.), whose times 

 of coming and going are more or less regular, there are numerous 

 winds which blow at irregular times, and whose coming cannot be 

 foretold long in advance. These irregular winds are the chief 

 cause of the uncertain elements of the weather. Some of them 

 are due to unequal temperatures, some to unequal amounts of 



