A controversial issue in climate science is the possible influence of Galactic Cosmic Ray flux (GCR) on clouds. Here, we present the results of a study to quantify the relationship between cloud cover and short-term Forbush decreases (FD) of GCR. FD events represent a useful sampling basis for studying GCR/cloud relationships as there are no known modes of internal climate variability operating at these timescales. Using an extensive record of global satellite derived cloud products from the International Satellite Cloud Climatology Project (ISCCP) D1 data series, epoch superposition analysis of a sample of FD events is conducted. This analysis is conducted at a range of spatial scales. Resulting mean cloud anomalies, relative to the period prior to FD event onset, are tested for significance using a randomised Monte Carlo experiment. The analysis reveals a substantial and significant cloud decrease, following FD event onset, in the high latitudes of the southern hemisphere. These anomalies occur largely in the high-level cloud, and are particularly pronounced (up to -30%) in the uppermost cloud (occurring at 10-180mb) over Antarctica. For corroboration of this finding the same methodology is applied to observations of surface temperature, upper air temperature and satellite derived estimates of polar stratospheric clouds.