Elements

ELEMENTS Temperature.—The rate of change of temperature with height is of fundamental importance in determining the processes of weather. The observations of Teisserenc de Bort showed that up to considerable heights the temperature decreases steadily with height, at an average rate of about 6° C per km., or 3° F per 1,000 ft., but that a limit is eventually reached beyond which the temperature remains constant, or even increases slightly at first. The lower region in which the temperature decreases with height is known as the troposphere, the upper region of vertically uniform temperature is known as the stratosphere, and the sur face separating the two is known as the tropopause. Observations have now been obtained from a sufficient number of stations distributed over the earth to show that the tropopause is higher at the equator than at the poles, being on the average about 18 km. high at the equator, 6 km. high at the pole in summer, and coming down nearly to the earth's surface in winter. In inter mediate latitudes, the height has intermediate values, the average height over southern Europe being given as 10.5 to 11 km. by W. H. Dines "The Characteristics of the Free Atmosphere," M.O. Geophysical Memoirs, No. 13.

A considerable amount of statistical work has been carried out by Dines (M.O. Geophysical Memoirs, No. 13), Schedler (Bei triige Phys. Jr. Atmos. Bd. vii.) and others, into the relations which exist between the temperature at different heights, the pres sures at different heights, the height of the tropopause, the tem perature of the stratosphere, the total water-vapour-content of the atmosphere, and various other factors.

Diurnal Variation of Temperature in the Upper Air.— Reference has already been made to the diurnal changes of tem perature on the Eiffel tower. Numerous observations have been made at greater heights, particularly a long series of kite sound ings by Assmann in Berlin from Oct. 1, 1902, to Dec. 31, 1903, and of kite balloon ascents at Lindenberg. (See Lindenberg Publi cations, vol. xiv., 1922.) Temperature is far more variable at a height of 6 or 7 km. than it is at the ground, but we can draw no conclusion as to diurnal variations at different heights from this result. (See also W. H. Dines, Q.J.R. Met. Soc., 1919; and K.

Mag. en Met. Obs. Batavia, V erhand No. 4, 1916.) Water Vapour.—Since temperature decreases with height, the saturation pressure of water vapour also decreases with height, and there is a finite limit to the amount of water vapour which the atmosphere can contain. The saturation pressure is independ ent of pressure, but on account of the rapid decrease of the den sity of dry air with decreasing pressure, we find that in saturated air the proportion of water to air is greater in the free air than it would be at the ground when at the same temperature.

The Variation of Wind with Height.

The variation of wind with height depends upon the horizontal distribution of temperature. It is therefore easy to realize that this variation cannot be summarized in a few words for the whole of the atmos phere. It is found that to a reasonable degree of approximation the motion of air is such as to produce a balance of the forces called into play, and this involves the motion of the air along the isobars, at all heights removed from the effects of surface friction. Near the ground the effect of friction is to diminish the velocity of the wind, causing the air to drift across the isobars from high to low pressure. It follows that in the lower layers the wind speed increases with height above the ground, while its direction veers The Fundamental Gas Equation.—The pressure p, the tem perature T, the density p or the specific volume v, are connected by the relations: where R is a constant. If the pressure is in millibars, the tem perature on the absolute centigrade scale, and the density in grams per cubic centimetre, or the specific volume in cubic centimetres per gram, the constant R is equal to 2.8703 X When the air contains water vapour, but not sufficient to saturate it, an equation of the same form may be used with a slightly different value of R; but this equation must then be used only at temperatures which are sufficiently high to ensure that the air is not saturated.