8o 



TROPICAL CLIMATOLOGY 



but at lesser altitudes it descends in the region of subtropical calms, 

 and either passes polewards or to the Equator. Within the area 

 from o to 30 degrees there are winds called the ' trades,' which 

 blow from 30 degrees towards the Equator, and which on reference 

 to Fig. 6, can be noted as north-east trades and south-east trades, 

 and it will also be observed that the only portion of the ocean 

 free from them is the North Indian Ocean, where the south- 

 west monsoons occur. The north-east monsoon is really a trade- 

 wind. 



Further, it will be noted that as the air moves in opposite direc- 

 tions along the surface in the region of latitude 35 degrees, there is 

 an absence of prevailing winds. These belts of calms are called ' the 



Fig. 7. — Diagram of the Circulation of the Atmosphere. 

 (After Ferrell.) 



subtropical belts,* or the calms of Cancer and Capricorn. Again, at 

 the Equator there are calms for the same reason, and these are often 

 called ' the doldrums.' 



The trade-winds are only to be seen typically in oceanic regions, 

 v/here the temperature is equable, and there are no local conditions 

 to cause them to deviate from their course. But they, together 

 with their intermediate zones of calms, shift their positions according 

 to the temperature, having, therefore, different areas in such months 

 as March and September. On an average the north-east trades 

 extend from 7 degrees to 29 degrees north in the Atlantic, and the 

 south-east to 20 degrees south. During the summer they advance 

 a few degrees north, arid in the winter recede to the south. In 

 spring the centre of the doldrums is only i to 2 degrees north of 



