A time-independent model is presented which predicts the short-lived radon daughter activity of rainwater at ground level
for unit concentration of radon (222Rn) in cloud. The model incorporates the physical processes of diffusive attachment of radon daughters and impact collection
for cloud droplets by raindrops. It also includes consideration of cloud droplet and raindrop size spectra. It is concluded
from the results from the model that the specific activity of rainwater is not affected very much by aerosol concentration,
liquid water content, average radius of raindrops and height of cloud base but is affected markedly by rainfall rate. The
radon concentrations in the rainclouds are estimated from the comparison between calculation and near-ground observations
carried out for a period of almost 3 years. The result shows that the monthly geometric mean concentration ranges from 2 to
6 pCi·m−3 at Nagoya.
Data of variations in the RaB/214Pb/ and RaC/214Bi/activities in precipitation obtained from observations made for a period of almost 5 years are given together with data on changes in precipitation rate measured simultaneously. It is concluded from analysis of these data that the precipitation rate variation contributes to approximately 40% to the seasonal variation in the specific activity of precipitation.