Hygrometry
The objective of atmospheric humidity measurements is to determine the amount of water vapor present in the atmosphere by weight, by volume, by partial pressure, or by a fraction (percentage) of the saturation (equilibrium) vapor pressure with respect to a plane surface of pure water. The measurement of atmospheric humidity in the field has been and continues to be troublesome. It is especially difficult for automatic weather stations where low cost, low power consumption, and reliability are common constraints. Pure water vapor in equilibrium with a plane surface of pure water exerts a pressure designated e's. This pressure is a function of the temperature of the vapor and liquid phases and can be obtained by integration of the Clausius-Clapeyron equation, assuming linear dependence of the latent heat of vaporization on temperature, L = L0(1+∝ (T-T0)], where T0 = 273.15K, L0 = 2.5008 x 106Jkg-1, the latent heat of water vapor at T0, Rv = 461.51Jkg-1K-1, the gas constant for water vapor, e's0 = 611.21 Pa, the equilibrium water vapor pressure at T = T0, and ∝ = - 9.477 x 10-4 K-1 = average rate of change coefficient for the latent heat of water vapor with respect to temperature. Since water vapor is not a perfect gas, the above equation is not an exact fit. The vapor pressure as a function of temperature has been determined by numerous experiments.