AbstractThe evaporation of a single drop of water as it falls through an atmosphere in which the lapse rate is 6·5°C/km and the surface temperature either 15°C or 41°C is examined. By making certain justified assumptions, it is shown that the change in surface area of a large drop (i.e., with radius exceeding 0·15 mm) as it falls from heightz1toz2is proportional to (1 —f)1·13exp (—lz) where 100fis the relative humidity andlis a constant. Values are given for the constant of proportionality and forl. If the drop has a radius less than 0·15 mm the change in volume of the drop is proportional to (1 —f)1·062exp (—lz).These results are then used to assess the effect of evaporation on the size and size distribution of raindrops falling through a constant atmosphere. It is shown that if the size distribution of raindrops at some initial height is given by 1 —F= exp — (2a/b)n whereFis the fraction of liquid water comprised by drops with radius less thana, andbandnare constants, then evaporation will lead to a change in the values ofbandnbut will not affect the general validity of the formula. As the rain falls through the non‐saturated air the distributive indexntends to a value between 3·5 and 4·0 whether it was greater or less than such a value initially. If the initial value ofnis less than 3·0 the scale diameterbincreases as a result of evaporation. If the initial value ofnexceeds about 3·5 the scale diameter decreases. The effect of evaporation upon the radar response from falling rain is also examined.In the last part of the paper the effect of the evaporation of rain into air which is initially unsaturated is considered. It is shown that the air temperature tends to a steady value and the relative humidity to 100 per cent. The excess of temperature above the steady‐state value and the deficit of relative humidity below 100 per cent decrease exponentially to zero with time. The depression of the steady‐state temperature below the initial temperature is a function only of the initial temperature and relative humidity and a table is given showing the value of the steady‐state temperature for nine different sets of initial conditions. The rate at which the final state is approached depends however only upon the rate of rainfall and a formula
展开▼
