Abstract
Measurements of the salinity of raindrops as a function of drop size show that there are characteristic variations according to the type of rain. For Bergeronβtype rain near Sydney, New South Wales, the dependence may be expressed in the form salinity Ξ± (radius)^βn^, where n is a positive constant which is small when the cloudbase is low and as high as 2Β·6 for collection well below the cloud. The mean salinities over all drop sizes range from 1 to 41 parts NaCl per million. The main features of the experimental results are explained by a theoretical treatment which takes into account evaporation and the collection of salt below the cloud.
For rain from warm clouds in the Hawaiian Islands, the salinity has a pronounced minimum at a certain drop size near the median. The daily mean salinities decrease with increasing distance inland, along the direction of tradeβwind flow, from 2Β·0β20 parts per million sodium chloride near sea level to 0Β·34β8Β·5 pt/mn near mean cloud base (2,100 ft) and 0Β·26β3Β·8 pt/mn at 4,500 ft. The salinity minimum occurs in samples at all three positions and secondary processes below the cloud cannot account for it; nor can it be a result of averaging the considerable variations of salinity which are found to occur from shower to shower.
It is suggested as a possible explanation that the largest cloud drops might form on giant salt nuclei and grow to raindrop sizes first by coalescence with one another. This receives support from measurements on single showers, which show that the first raindrops to fall are large and have a high salinity. A detailed interpretation would seem to require a knowledge of the salt concentration in individual cloud drops.