8 THE AIR 



extends far over the sea, and introduces many com- 

 plications of normal climate due to geographical 

 position. The general effect of the continents is to 

 break up the belts of high and low pressure round the 

 world, the high-pressure belts being most nearly 

 continuous in the winter months, the low-pressure 

 belts most nearly continuous in the summer months, 

 of the respective hemispheres. 



The alternate heating and cooling of continents in 

 summer and winter gives rise to monsoons, or seasonal 

 winds, blowing from the sea towards the lowered 

 pressure over the heated land in summer, and from 

 the high pressure over the chilled continents towards 

 the warmer sea in winter. Monsoon winds occur on 

 all continental coasts, but the monsoons attain their 

 greatest development in the Indian Ocean, where Asia, 

 Africa, and Australia combine in modifying the 

 theoretical wind-directions. Over Asia the monsoon- 

 producing conditions exist in great intensity. The 

 result is that the south-east trade in the northern 

 summer is drawn northward across the obliterated 

 belt of equatorial calms and converted into a south- 

 westerly wind, called the " South-west Monsoon," 

 which, in many places, blows with the force of a gale. 



In the northern winter the north-east trade-wind is 

 greatly strengthened, and is called the " North-east 

 Monsoon," but it never attains the force of the south- 

 west monsoon. At the changing of the monsoons, 

 approximately at the time of the equinoxes, disturbed 

 conditions, giving rise to much bad weather, are apt 

 to occur. 



Land and sea breezes, with a diurnal period of 

 alternation, occur in similar conditions to monsoons, 

 but on a smaller scale, as they are due to the heating 

 of the land by day and its cooling by night. The sea- 



