78 



TROPICAL CLIMATOLOGY 



PRESSURE. 



The atmospheric pressure at different localities scattered all 

 over the world may be measured by the barometer, due care being 

 taken to correct the reading for index error, capacity, capillarity, 

 temperature, and altitude ; and the data so obtained may be entered 

 on a map of Mercator's projection of the world, and finally points 

 of similar pressure, as indicated in this map, may be joined together 

 by curves, thus producing an isobaric chart. 



The result of this investigation shows that, generally speaking, 

 at sea-level there is high pressure at about 30 degrees north or south 

 of the Equator, and that from this the pressure decreases towards 

 the Equator. 



The reason why the pressure is low at the Equator is partially 

 due to the heating of the air by the sun's rays, but more especially to 

 the considerable addition of aqueous vapour to the air in these 

 regions, with the result that it ascends with considerable force, 

 and goes on ascending to very high altitudes, owing to repeated 

 warming from the condensation of aqueous vapour and the liberation 

 of latent heat. 



Though the pressure at 30 degrees is higher than that at 

 the Equator at sea-level, it is considerably less than at higher 

 altitudes. Consequently air passing upwards from sea-level at 

 the Equator into higher strata will then flow either north or south 

 towards 30 degrees; but as it flows it is compressed as the latitude 

 increases, and hence the current of air flowing polewards becomes 

 narrower and narrower, and finally, owing to this, is forced down 

 by the increasing pressure, until it reaches sea-level at 30 degrees 

 north or south, causing the high pressure about this latitude. 

 This compression is helped by the cooling of the air as it proceeds 

 polewards, and by the centrifugal force of the earth's rotation. 



Apart from this general distribution, there are diurnal and annual 

 variations in the pressure. The diurnal variation of pressure is 

 best marked in the tropics, diminishing as the Poles are approached 

 — e.g., at Calcutta it is o-i2 inch, whereas at Greenwich it is only 

 0-02 inch. The cause of this diurnal variation is the heating of the 

 air by the sun, but there is also another cause, producing, according 

 to Blandford, a variation of o-i inch in India, and acting twice 

 daily, the nature of which is not known. 



The cause of these seasonal variations in the pressure is the 

 heating or cooling of the land, which, in the case of Colombo, is 

 associated with the monsoons. Thus, the low pressures occur in 

 the season of the south-west monsoon. May, June, and July, 

 when air is travelling from high pressure at the Equator to low 

 pressure in the warm plains of Asia; and the high pressures are 

 associated with the north-east monsoon, when air is travelling 

 from the cold plains of Asia, where it is at considerable pressure, 

 towards the Equator, where the pressure is less. 



