The chemical composition of precipitation at cloud levels

In an attempt to obtain precipitation samples free from contamination either from industrial sources or from sea spray, samples were collected by an aircraft of the Meteorological Research Flight over Southern England at heights up to 8,500 ft. Most of the samples still showed clear evidence of industrial pollution: however, five of them were collected under synoptic conditions which render pollution improbable. The amount of dissolved matter in these samples is much smaller than in the remainder, and their composition shows a close similarity with that of samples collected at ground level in regions where pollution is slight. It is considered likely that the solute has been little affected by matter derived directly from land : but it still shows characteristic differences from sea‐salt, particularly in the much smaller proportions of sodium and chlorine in comparison with all other elements. It is suggested that these differences must arise from some reaction, occurring either at the sea surface or in the atmosphere itself, but there is insufficient evidence to determine its nature.