The Gas Constant, R.—The equation of condition for gases is where p is the pressure, v the volume of unit mass, T the abso lute temperature, and R is a constant. If the pressure is expressed in millibars, and v is in cubic metres per kilogram the value of R for dry air is 2.8703.
The Density of Damp Air. Virtual Temperature.—For damp air at a total pressure p and vapour pressure e, the total density is the sum of the densities of the dry air and water vapour. The density is therefore Geopotential.—It has been suggested that the height of a point above the surface of the earth should be defined by the geopotential, or the potential energy of unit mass at that height due to its elevation above the surface of the geoid. Surfaces of equal geopotential are "level surfaces" and are horizontal in the technical sense.
The geopotential being the potential energy of unit mass, has the dimensions of On the c.g.s. system the unit is I A convenient unit for ordinary working purposes is the dynamic metre, equal to The approximate rela tion of the dynamic metre to the metre for latitude 5o° is dynamic metre= I.o2o9 metre. Tables of geopotential are given in Bjerknes, Dynamical Meteorology and Hydrography.
For full details of the types of instrument which are in normal use in meteorological observations, reference should be made to the Meteorological Observers Handbook of the Meteorological perature will be found in Bjerknes' Dynamical Meteorology and Hydrography, vol. ii. (Smithsonian Institution).
sorption is that of Hettner (Ann. d. Phys. 1905), who found that there is very marked absorption between 4.4 and 8 bc and above 12 while there is a band from 8 ,u to II ,u within which there is only slight absorption by water vapour. Radiation from the earth at normal temperatures has a maximum intensity at about io and so the existence of the band of relative parency from 8 ,u to II ti is of fundamental importance. The elementary gases absorb no radiation at atmospheric tempera tures.
The concept of potential temperature is particularly useful in dealing with conditions of vertical stability in the atmosphere. It was first introduced by von Bezold (Sitz. Ber. Akad, 1888).
Entropy.—A definition of entropy will be found in the article THERMODYNAMICS. If a small amount of heat dQ is communi cated isothermally (i.e., without changing the temperature) at a temperature T to a mass of air, then the ratio a small change of entropy, and is denoted by c/0 (0 being the symbol for entropy). Thus, in an isothermal process, the gain or loss of heat is represented by Td4. Entropy remains constant for any adiabatic changes (i.e., changes where heat is neither lost nor absorbed by the system). If we take zero of 4) as corresponding to a potential temperature of Ioo° on the absolute scale, Office, or any standard textbook on the subject. (See also BA ROMETRY, THERMOMETRY, etc.) Air temperature does not readily permit of accurate determina tion. Difficulties arise through the necessity for ventilation of the instruments, combined with adequate protection from direct radiation from the sun or surrounding objects, combined with the variability of temperature between adjacent masses of air. The accuracy of determination of air temperature is therefore much inferior to the accuracy of the determination of the temperature of a liquid in the laboratory. This is but one example of the very real difficulties of accurate meteorological observation, and the existence of these difficulties must be borne in mind in what follows.
The methods which have hitherto been developed for upper air observations are only applicable in relatively fine weather, and so little is known from actual observation of the conditions at high levels above cyclones. This is probably the main reason for the slow advance made in meteorology during the last half-century.