SATURATION AND COOLING OF AIR LAYERS BY EVAPORATION FROM FALLING RAIN

Abstract

The changes in temperature and relative humidity caused by evaporation from raindrops are computed for several cases involving three different lapse rates and two rainfall intensities. Formulae for evaporation from stationary drops are modified to include the effects of relative velocity and continuously changing air temperature. It is found that saturation, due to evaporation alone, will not be produced in an isothermal layer or a layer having a decrease of temperature with height. In the case of the isothermal layer, the air is very close to saturation after six hours. The temperature decrease during the same interval is 4.5°C. Due to the lower equilibrium drop temperature, the final relative humidity in a layer having a moist-adiabatic lapse rate is near 97 per cent and the temperature fall is slightly smaller. In an inversion, however, the humidity reaches 100 per cent in two and one-half hours and approaches 107 per cent after six hours. The excess moisture present is sufficient to produce fog or clouds. In general, evaporation is greatest in the warmest part of a layer; the change in relative humidity is greatest in the coolest part. Thus, the ceiling would build down from above in a layer having a normal lapse rate provided other influences tending to produce condensation are present. Inversions favor the formation of fog or low stratus due to evaporation alone.