What was done
The authors note that because the net global effect of cloud is cooling (Hartman, 1993), any widespread increase in the amount of overcast days could reduce air temperature globally, while local overcast conditions could do so locally. Hence, they compared the ratio of diffuse to total solar radiation - the diffuse fraction (DF) - which had been measured daily at 0900 UT at Whiteknights, Reading (UK) from 1997-2004, with the traditional subjective determination of cloud amount made simultaneously by a human observer, as well as with daily average temperature. They then compared the diffuse fraction measured at Jersey between 1968 and 1994 with corresponding daily mean neutron count rates measured at Climax, Colorado (USA), which provide a globally representative indicator of the galactic cosmic ray flux.

What was learned
Harrison and Stephenson report that "across the UK, on days of high cosmic ray flux (above 3600x10² neutron counts per hour, which occur 87% of the time on average) compared with low cosmic ray flux, (i) the chance of an overcast day increases by 19% ± 4%, and (ii) the diffuse fraction increases by 2% ± 0.3%." In addition, they found that "during sudden transient reductions in cosmic rays (e.g. Forbush events), simultaneous decreases occur in the diffuse fraction."

What it means
The two researchers note that the latter of these observations indicates that diffuse radiation changes are "unambiguously due to cosmic rays." They also report that "at Reading, the measured sensitivity of daily average temperatures to DF for overcast days is -0.2 K per 0.01 change in DR." Consequently, they suggest that the well known inverse relationship between galactic cosmic rays and solar activity (see Cosmic Rays in our Subject Index) will lead to cooling at solar minima, and that "this might amplify the effect of the small solar cycle variation in total solar irradiance, believed to be underestimated by climate models (Stott et al., 2003) which neglect a cosmic ray effect." In addition, although the effect they detect is small, they say it is "statistically robust," and that the cosmic ray effect on clouds likely "will emerge on long time scales with less variability than the considerable variability of daily cloudiness."

References
Hartman, D.L. 1993. Radiative effects of clouds on earth's climate. In: Hobbs, P.V. (Ed.), Aerosol-Cloud-Climate Interactions. Academic Press, New York, New York, USA.

Stott, P.A., Jones, G.S. and Mitchell, J.F.B. 2003. Do models underestimate the solar contribution to recent climate change? Journal of Climate 16: 4079-4093.