  C.T.R.Wilson showed that when supersaturated water vapour was exposed to
ultraviolet radiation from sunlight or other sources in the presence of oxygen
it immediately condensed to form an aerosol. This phenomenon was eventually
explained as due to the formation of a charge-transfer complex H2O+O2-, whose
existence was confirmed theoretically and subsequently established
experimentally. It is proposed that the correlation recently discovered between
the stream-flow of the Parana and three other rivers in South America and the
solar sunspot cycle is due to the photo-nucleation mechanism investigated by
Wilson.
