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The response of clouds to sudden decreases in the flux of galactic cosmicrays (GCR) – Forbush decrease events – has been investigated using cloudproducts from the space-borne MODIS instrument, which has been in operationsince 2000. By focusing on pristine Southern Hemisphere ocean regions weexamine areas where we believe that a cosmic ray signal should be easier todetect than elsewhere. While previous studies have mainly considered cloudcover, the high spatial and spectral resolution of MODIS allows for a morethorough study of microphysical parameters such as cloud droplet size, cloudwater content and cloud optical depth, in addition to cloud cover. Averagingthe results from the 22 Forbush decrease events that were considered, nostatistically significant correlations were found between any of the fourcloud parameters and GCR, when autocorrelations were taken into account.Splitting the area of study into six domains, all of them have a negativecorrelation between GCR and cloud droplet size, in agreement with a cosmicray – cloud coupling, but in only one of the domains (eastern AtlanticOcean) was the correlation statistically significant. Conversely, cloudoptical depth is mostly negatively correlated with GCR, and in the easternAtlantic Ocean domain that correlation is statistically significant. Forcloud cover and liquid water path, the correlations with GCR are weaker,with large variations between the different domains. When only the sixForbush decrease events with the largest amplitude (more than 10%decrease) were studied, the correlations fit the hypothesis slightly better,with 16 out of 24 correlations having the expected sign, although many ofthe correlations are quite weak. Introducing a time lag of a few days forclouds to respond to the cosmic ray signal the correlations tend to becomeweaker and even to change sign

Cosmic rays, cloud condensation nuclei and clouds – a reassessment using MODIS data