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sunrise on the island the air is still. As the sun gets hotter, 
the land and the air upon it are more rapidly warmed than 
the sea and the air in contact with its waters, and a wind 
begins to blow in from the sea. The sea-breeze becomes 
stronger as the day goes on, reaching its maximum force at 
about 3 p.m. As the sun’s power decreases, the ground gives 
up its heat much more readily than the water of the sea, 
and as the evening approaches the air over the land becomes 
cooler than that over the neighbouring sea, so that a wind 
is set up blowing from the land out to sea, which continues 
until the slowly-cooling sea has fallen to the same temperature 
as the land. This process is repeated daily with the utmost 
regularity. 
In some parts of the world the winds are regular and 
constant all the year round. The earth receives the directest 
and hottest of the sun’s rays at the equator, and at the equator 
becomes warmer than elsewhere, so that the air at the equator 
is continually rising, cold air from the north and south taking 
its place and forming the trade-winds blowing towards the 
equator from the north-east and south-east. The reason 
that these winds do not blow directly south and north is that 
their initial eastward speed, taken from the eastward rotation 
of the earth, remains constant, while as they travel the earth’s 
belt is growing constantly wider and wider and the earth’s 
surface moving faster and faster, since things nearer the 
equator have farther to go in 24 hours than things more 
remote from it. The winds, therefore, keep dropping behind 
the eastward-flying surface of the earth, and instead of coming 
towards the equator directly from the north and south 
approach it from the northern hemisphere in a south-westerly, 
and from the southern hemisphere in a north-westerly direction. 
In many places the configuration of land and sea causes 
irregularities in the atmospheric movements. In the Indian 
Ocean, for instance, the trade-wind periodically reverses 
itself owing to the proximity of the great land-mass of Asia. 
During the summer the Asiatic continent becomes intensely 
hot compared with the waters of the Indian Ocean, and as a 
result a current of cokl air, known as the south-west monsoon, 
flows from the south-west across India between April and 
October. 
Zones in which there is very little circulation of the atmos- 
phere occur round the earth in different latitudes. One 
immediately north and south of the equator is known as the 
Doldrums, and there are similar zones at the tropics of Cancer 
and Capricorn. 
