"Good Day Sunshine: Stock Returns and the Weather," David Hirshleifer and Tyler Shumway, March, 2001. The authors are at Ohio State University and the University of Michigan, respectively. They explain that:

Psychological evidence and casual intuition predict that sunny weather is associated with upbeat mood. This paper examines the relation between morning sunshine at a country's leading stock exchange and market index stock returns that day at 26 stock exchanges internationally from 1982-97. Sunshine is strongly positively correlated with daily stock returns. After controlling for sunshine, other weather conditions such as rain and snow are unrelated to returns. If transactions costs are assumed to be minor, it is possible to trade profitably on the weather. These results are difficult to reconcile with fully rational price-setting.

The full report is published on-line at [link|http://www-personal.umich.edu/~shumway/papers.dir/weath15.pdf|<[link|http://www-personal.umich.edu/~shumway/papers.dir/weath15.pdf|http://www-personal.../weath15.pdf]>]