The transport and deposition of debris from nuclear test explosions

Fine debris from nuclear test explosions is removed from the atmosphere by dry deposition or by rainwater. The relative importance of these two mechanisms is considered and the conclusion is reached that, in middle latitudes, at least 80% of the fision product activity in soil has been brought down in rain. The average life of a small particle in the troposphere is deduced from the rate of decrease of air concentration and from the relation between air and rainwater activities and lies in the range 30–60 days. The factors influencing the attachment of a particle to a raindrop are discussed in the light of the most recent data and tentative conclusions are reached on the comparative efficiences of the three processes:

1. 1.
   Attachment to cloud droplets by Brownian motion.