Solar Activity and Cloud Opacity Variations: A Modulated Cosmic-Ray Ionization Model

The observed correlation between global low cloud amount and the flux of high energy cosmic-rays supports the idea that ionization plays a crucial role in tropospheric cloud formation. We explore this idea quantitatively with a simple model of cosmic-ray ionization enhancement of the formation of cloud condensation nuclei. This model predicts that solar modulation of the cosmic-ray ionization rate should be correlated with cloud opacity where the atmospheric aerosol concentration is low. Using the International Satellite Cloud Climatology Project database (1983-1993), we find that the mean opacity of low latitude (< 40◦) clouds does indeed show a correlation with the variations in cosmic-ray flux, and that the observed opacity variations increase with altitude in accordance with the model. We also find that the higher latitude (> 40◦) clouds, on the other hand, show an anti-correlation with cosmic-ray flux, which we suggest may be a feedback effect resulting from the thicker low latitude clouds. We also show that the previously reported correlations of cloud amount with cosmic-ray flux probably result from the variations in longwave emissivity expected from our model, and not from variations in cloud amount. Further global cloud observations by missions such as Triana are needed to better study the apparent variations of cloud opacity with cosmic-ray ionization rate and solar activity.