Atmospheric Salt Particles and Raindrops

Marshall and Palmer (1948) have shown that, for rains of a given intensity, there is a definite distribution curve of number of raindrops of a particular range of diameter. The writer has indicated here that the chlorinity of rains also varies with rain intensity. Recently obtained data, concerning atmospheric sea salt, are presented in the form of distribution curves. These curves show the number of sea-salt particles sampled at different altitudes, of a given weight range, plotted against the weight. A computation is made, using a salt-particle distribution curve obtained at cloud levels, in which water is added to each particle until it reaches an assumed chlorinity for a given rain intensity. Each particle is thereby increased in size and becomes a drop of a new weight. The distribution curves of these computed drops are compared to the observed distribution curves of Marshall and Palmer, for various rain intensities, and are found to be remarkably similar. This result implies that, in the process of growth, the droplets containing each salt particle grow to raindrop size through coalescence with much more numerous and relatively non-saline cloud droplets. The numbers of droplets in cumulus clouds over the sea are compared to the numbers of condensation nuclei in the sub-cloud layer and to the number of larger sea-salt particles. A method of sampling the large sparsely-distributed salt particles in the atmosphere is described briefly.