Turbulent Fluctuations of Humidity and Their Measurement

The turbulent transfer of water vapor in the lowest atmospheric layers is the principal mechanism of energy exchange between air and sea and plays an important part in problems of electromagnetic‐wave propagation. Since experimental study of this transfer requires the measurement of humidity fluctuations, their characteristics have to be estimated to delineate suitable measuring devices. This estimate is obtained from the following information: (a) Equations governing the mean distributions, turbulent fluxes, and levels of fluctuations of humidity. These expressions bring out the analogy existing between humidity and temperature and enable the identification of various mechanisms of generation, transfer, and dissipation inside the surface layer of a stratified atmosphere. (b) Experimental results for the mean profile, the transfer rate, and the turbulence level of water vapor in typical cases. (c) Predictions of the spectral distribution and of time and space scales of the turbulence of humidity. Once the estimate is made, it is then possible to define specifications for devices measuring humidity fluctuations for use in either the field or the laboratory. Various methods of measurement can be considered; i.e., psychrometry, measurement of dew‐point, use of the hot wire, absorption of Lyman–α or infrared radiation, measurement of refractive index, or use of the ionic probe. Although small time‐constant psychrometers offer some interest, electromagnetic techniques appear most promising.