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 linking the concentration of cloud condensation nuclei to the varying ionization rate due to cosmic rays. Among the predictions of the model is a variation in global cloud optical thickness, or opacity, with cosmic-ray rate. Using the International Satellite Cloud Climatology Project database (1983-1999), we search for variations in the yearly mean visible cloud opacity and visible cloud amount due to cosmic rays. After separating out temporal variations in the data due to the Mt. Pinatubo eruption and El Nino/Southern Oscillation, we identify systematic variations in opacity and cloud amount due to cosmic rays. We find that the fractional amplitude of the opacity variations due to cosmic rays increases with cloud altitude, becoming approximately zero or negative (inverse correlation) for low clouds. Conversely, the fractional changes in visible cloud amount due to cosmic rays are only positively-correlated for low clouds and become negative or zero for the higher clouds. The opacity trends suggest behavior contrary to the current predictions of ion-mediated nucleation (IMN) models, but more accurate temporal modeling of the ISCCP data is needed before definitive conclusions can be drawn. 
David Marsden (1) and Richard E. Lingenfelter (2) ((1) Scripps   Institute of Oceanography, (2) UCSD)