Page:Handbook of Meteorology.djvu/13



Meteorology is the science of the air. The air is the outer shell, or layer, of the earth; hence it is called the atmosphere, a Greek word meaning “air-sphere”. The movements of the air (the winds) and the variable proportion of water vapor, mingled with and forming a part of it, exert a profound influence which affects the climate, habitability and civilization of the earth.

For the greater part the movements of the air and the variations in the proportion of moisture are the results of changes in temperature. The fundamental study of the physics of the air, therefore, concerns the problems of variations in temperature and the far-reaching results of those variations.

Composition. — The atmosphere consists of a mixture of gases of which oxygen, nitrogen and the argon group constitute about 98 per cent. The composition varies very slightly so far as these are concerned; but the proportion of water vapor and of the other components varies materially. The first of the two tables which follow is the average of many analyses made by Rayleigh and Ramsay; the second is on the authority of Humphreys. Other analyses show slight differences that indicate actual differences in proportion rather than errors in analyses.