The search for cosmic ray effects on clouds

The cause of the correlation of low cloud cover with the sunspot cycle, and the associated cosmic ray intensity, is still the subject of controversy. Insofar as ‘clouds’ come in different types with, doubtless, different sensitivities to the cloud condensation nuclei (charged or otherwise) it is useful to search for differences in the correlation between cloud types. Here, we examine the major cloud components: stratiform and cumuliform, the latter with its much higher upthrust velocities being expected to be less efficient as cosmic ray induced cloud generation. No difference is found between the two types of cloud, in the sense that there is no dependence on the fraction of cloud of stratiform type for the various parameters studied. This result is true over the Globe as a whole and as a function of cosmic ray cutoff rigidity. Once more there is no support for the cosmic ray hypothesis for cloud modulation. There is no obvious implication for the alternative hypotheses of solar irradiance modulation, but this is probably still the more likely.