The theory of condensation nucleation applied to radioactive fallout constituents shows that most tropospheric fallout particles are efficient nuclei in the production of warm rain. As a result it is found that the concentration of fallout in precipitation is a maximum soon after the onset of rain. This is supported by measurements made during many individual storms over an extensive period. The influence on fallout concentration of variations in rainfall rate through a storm is found to be complicated by the opposing effects of the liquid water content of the cloud and the generation of downdrafts within it.