Laboratory investigation of possible ice nucleation by ionizing radiation in pure water at tropospheric temperatures

A variety of mechanisms have been proposed to explain observed correlations between tropospheric climate variables and cosmic ray flux levels. The mos t direct hypothesis involves the formation of ice nuclei along ion tracks f ormed when cosmic rays pass through cloud water droplets. To test this hypo thesis, we performed nucleation studies of a single water droplet undergoin g repeated linear cooling cycles to -40 degreesC with and without a radioac tive source in the vicinity of the drop. A comparison of the distributions of nucleation temperatures obtained with and without a source of 5.3 MeV cy -particles present supports the null hypothesis that ionization tracks do n ot act as nucleation sites. In both cases we observe mean nucleation temper atures of -34.5 degreesC consistent with previous studies of homogeneous nu cleation.