To investigate whether galactic cosmic rays (GCR) may influence cloud cover variations, we analyze cloud cover anomalies from 1900--1987 over the United States. Results of spectral analyses reveal a statistically significant cloud cover signal at the period of 11 years; the coherence between cloud cover and solar variability proxy is 0.7 and statistically significant with 95% confidence. In addition, cloud data derived from the NCAR Climate System Model (CSM) forced with solar irradiance variations show a strong signal at 11 years that is not apparent in cloud data from runs with constant solar input. The cloud cover variations are in phase with the solar cycle and not the GCR. Our results suggest that cloud variabilities may be affected by a modulation of the atmospheric circulation resulting from variations of the solar-UV-ozone-induced heating of the atmosphere. ¿ 2001 American Geophysical Union