  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.
