The Freakonomics duo has an interesting column on the use of weather by empirical economists. It has been used as an instrumental variable in several cases. It is likely to be correlated with the included variable but uncorrelated with the error, or the excluded variable.
It was used in the autism study.
I'm thinking the weather might be a good instrument for some sex research. Under the assumption that bad weather leads to staying indoors, and more sex among married couples and cohabitants, but fewer matching opportunities for single people.