Table 93 347 



DEFINITIONS AND SPECIFICATIONS OF WATER VAPOR IN THE 



ATMOSPHERE 1 



The Conference of Directors, International Meteorological Organization, Washington, 

 1947, decided to adopt the following definitions and specifications of the parameters of 

 water vapor in the atmosphere : 



(1) The mixing ratio r of moist air is the ratio of the mass m v of water vapor to the 

 mass m a of dry air with which the water vapor is associated. 



r = m v /m a 



(2) The specific humidity, mass concentration or moisture content q of moist air is 

 the ratio of the mass m v of water vapor to the mass (m» + ««) of moist air in 

 which the mass of water vapor m„ is contained. 



Q = - 



m v + m* 



(3) Vapor concentration (density of water vapor in a mixture) or absolute humidity. — 



For a mixture of water vapor and dry air the vapor-concentration dv is defined 

 as the ratio of the mass of vapor m v to the volume V occupied by the mixture. 



V 



(4) The vapor pressure e' of water vapor in moist air at total pressure p and with 

 mixing ratio r is defined by : 



e'= r - p 



0.62197 + r V 



(5) Saturation. — Moist air at temperature T and at total pressure p is said to be satu- 

 rated if its composition is such that it can coexist in neutral equilibrium with a 

 plane surface of pure condensed phase (water or ice) at the same temperature 

 and pressure. 



(6) Saturation mixing ratio. — The symbol r w denotes saturation mixing ratio of 

 moist air with_ respect to a plane surface of pure water. The symbol n denotes 

 saturation mixing ratio of moist air with respect to a plane surface of pure ice. 



(7) Saturation vapor pressure in the pure phase. — The saturation vapor pressure ev 

 of pure aqueous vapor with respect to water is the pressure of the vapor when in 

 a state of neutral equilibrium with a plane surface of pure water at the same 

 temperature and pressure ; similarly for d in respect to ice. ev> and t?t are tempera- 

 ture-dependent functions only; i.e. 



e« = ew(T) 

 £♦ = ei(T) 



(8) Saturation vapor pressure of moist air. — The saturation vapor pressure with 

 respect to water e' w of moist air at pressure p and temperature T is defined by : 



0.62197 + r« r 



Similarly, the saturation vapor pressure with respect to ice e\ of moist air at 

 pressure p and temperature T is defined by: 



e ♦ = ■ 



0.62197 + r« r 



(9) Relations between saturation vapor pressures of pure phase and of moist air. — 



In the meteorological range of pressure and temperature the following relations 



hold with an error of 0.5 percent or less : a 



j „ 



e 40 ^— t?to 



e\ = t?i 

 (10) The thermodynamic dew-point temperature Ta of moist air at temperature T, 

 pressure p and mixing ratio r is the temperature to which the air must be cooled 

 in order that it shall be saturated with respect to water at the initial pressure p 

 and mixing ratio r. 



1 Resolution 166, International Meteorological Organization, Conference of Directors, Washington, 

 1947. 



2 See Tables 89 and 90 for the exact relationship. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



