The Science of the Weather 769 



by the snow-capped mountains even in Equatorial regions, it will 

 give us food for reflection to remember that all the teeming life 

 on our planet is rendered possible by a thin zone of air approxi- 

 mately three miles deep at the Equator, and thinning out north- 

 ward and southward till it ends at the ground-level in the neigh- 

 bourhood of the Arctic and Antarctic Circles. Of the whole 

 extent of our atmosphere this belt or zone is all that has a 

 temperature above freezing-point. 



3 



Pressure and Temperature 



Atmospheric disturbances, then, have their origin in changes 

 of temperature and pressure. Temperature is a fundamental 

 element in Meteorology. The surface of the earth derives its 

 warmth almost entirely from the sun. The rays of light and heat 

 which emanate from the sun fall upon the surface of the earth, 

 causing an increase in its temperature. As there are great differ- 

 ences in the nature of the earth's surface, there are consequently 

 great differences in the warming effect produced. Land, for 

 instance, heats up very rapidly, while water becomes warmed 

 much more slowly. The air is not heated by the passage through 

 it of the sun's rays, but by contact with the surface of the earth. 

 The land and water surfaces communicate their heat to the air 

 lying above them, and so in summer, and at midday, the air over 

 the land will be much warmer than that over the neighbouring 

 ocean. But, on the other hand, the water does not part with its 

 heat so readily as does the land, and therefore in winter, and at 

 night, the air over the land is generally colder than that over the 

 ocean. 



The atmosphere is, as we have already stated, a mixture of 

 gases, and all gases possess certain properties in common. When 

 heated, gases expand in volume, and when cooled, they contract. 

 If a gas is expanded, or, in other words, if its volume is increased, 

 it becomes cooler, and if compressed its temperature increases. 



