The Composition and Some Physical Properties of the Atmosphere

THE COMPOSITION AND SOME PHYSICAL PROPERTIES OF THE ATMOSPHERE General Composition.—The atmosphere is a simple mixture of gases, of which nitrogen and oxygen account for a little more than 99%. The proportions of the usual constituents of dry air are given in a table in the article ATMOSPHERE.

Constitution at High Altitudes.

In view of its bearing on theories of the origin of aurorae, magnetic storms and other phe nomena, considerable interest attaches to the constitution of the atmosphere at higher levels, say from 20 km. to 200 km. Com putation of the constitution of the atmosphere at these levels is based on the effects of diffusion, the convective mixing which is effective in producing constancy of composition in the lower layers having no effect at the higher levels. Each constituent is assumed to follow its own law of variation of density indepen dently of the others, in accordance with Dalton's law. (See CHEM ISTRY : Physical.) The lighter constituents will therefore tend to predominate more and more with increasing height. Hence it is of the utmost importance to ascertain what are the normal constituents of the atmosphere. If hydrogen is assumed to form a normal constituent of the atmosphere, then at great heights hydrogen will be the predominating constituent of the atmosphere.

In Wegener's scheme nitrogen ceases to be measurable at about 00 km. and beyond this level the composition of air by volume is about 5% helium, and 95% hydrogen and geocoronium. Chapman and Milne (Q.J.R. Met. Soc. vol. xlvi.) regard the hydrogen observed at low levels as accidental, so that the lightest normal constituent of the atmosphere is helium. Experimental data are insufficient to decide between the different schemes of constitu tion of the high level atmosphere. According to Dobson (Q.J.R. Met. Soc., 1923) meteors indicate that oxygen and nitrogen are the chief constituents up to about 16o km. McLennan attributes the green auroral line, of wave-length X=5577.35 A.U. (5.57735 cm., or .557735 1.4) to atomic oxygen. These re

sults appear to conflict with those derived from the assumption of Dalton's law.

Aqueous Vapour in the Atmosphere. Humidity.—The amount of aqueous vapour present in unit volume of air varies within wide limits from place to place, and from time to time in a given place. It may account for anything from o to 2.5% of the weight of a specimen of air. The saturation vapour pressure of air depends only on the temperature of the air, and is inde pendent of the total pressure. If e is the saturation vapour pres sure at temperature t° C, then it should be possible to represent e as a function of t. No general formula has ever been discov ered, though a large number of empirical formulae have been proposed by various writers. (See T. Preston Theory of Heat, v., 3rd ed. 1919.) The relative humidity, vapour pressure, etc., are normally de termined by observations of wet and dry bulb thermometers. If t and t' are the readings of the dry and wet bulb thermometers respectively, p the pressure, e' the saturation vapour pressure at the temperature of the wet bulb t', and e is the vapour pressure of the air under observation, these quantities are related by the formula where A is a constant. The value of A depends upon the degree of ventilation of the thermometers, and care must be exercised to use the value of A appropriate to the conditions.

Tables will be found in Jelinek's Psycrometer Tafeln, in the Smithsonian Meteorological Tables, and in the Humidity Tables published by the Meteorological Office, London (M.O. 265).

Physical Constants for

Air.—The density of dry air con• taining no CO2, as determined by Regnault at the College de France, is 1.29321 kg. per cubic metre, at o° C and at a pres sure of 76o mm. At a pressure of i,000 millibars and o° C, the corresponding figure is 1.2759o. Dry atmospheric air at a tem perature of o° C and i,000 mb. has a density of 1.27617. The ratio of the density of water vapour to that of dry air at the same temperature and pressure is 0.6221, approximately 1.