42 PLANT GEOGRAPHY 



a rainy season when it does so, and a dry season during 

 the rest of the year; while near the Equator, where it 

 traverses the region both in its northward and in its 

 southward swing, two wet and two drier seasons can be 

 recognised. 



The name monsoon, or season, was originally applied 

 to that wide-spreading system of semi-annually reversed 

 circulation which owes its origin to the vast concentra- 

 tion of land in Asia. During the winter, from October 

 to February, when the " continental " temperature of 

 Central Asia falls very low, and the barometer is pro- 

 portionately high, the north-east or winter monsoon 

 sweeps down over the Himalayas as a dry season, save 

 to the S.W. of the Bay of Bengal. In May the con- 

 ditions are reversed. High temperature and low pressure 

 begins to prevail in Central Asia. The monsoon, as it 

 is called in India, sweeps from the S.W. across the 

 Arabian Sea with heavy rain on the Western Ghats, 

 known as " the bursting of the monsoon." In the 

 Ganges valley it blows along the foot of the Himalayas 

 from the S.E.; and this wet monsoon continues till 

 August. These winds affect the currents of the northern 

 half of the Indian Ocean and the climate of all Asia, 

 Borneo. North-west Australia, and Madagascar. Similar 

 conditions occur on a smaller scale in the Great Basin of 

 the Rocky Mountains, in North Africa, Australia, and 

 the Spanish Peninsula'. 



Whilst winds may carry oceanic influences inland, as 

 in Western Europe, they may also carry continental 

 influences seaward, as off Eastern Asia. 



The drifting of sand by wind, especially on the coast 

 and in the arid areas of the tropics, produces the marked 

 conditions of sand-dunes and deserts, with their charac- 

 teristic vegetation, while the upward deflection of 

 atmospheric currents produced by mountain-chains 

 causes the precipitation of their moisture on the wind- 

 ward slope, and may markedly raise the temperature at 

 various levels as compared with corresponding levels on 

 the lee side. The high rainfall on the east side of the 

 Andes and of Australia and on the west of the Indian 

 Peninsula are examples of the one, whilst the warm 

 alpine valleys affected by a " fohn," or daily upward 

 wind, illustrate the latter case. 



Wind increases rapidly in velocity in proportion to 



