There is considerable empirical evidence that emotion influences decision making. In this paper, we use a database of individual investor accounts to examine the weather effects on traders. Our analysis of the trading activity in five major US cities over a six year period finds virtually no difference in individuals' propensity to buy or sell equities on cloudy days as opposed to sunny days. If the association between cloud cover and stock returns documented for New York and other world cities is indeed caused by investor mood swings, our findings suggest that researchers should focus on the attitudes of market makers, news providers or other agents physically located in the city hosting the exchange. NYSE spreads widen on cloudy days. When we control for this, the weather effect becomes smaller and insignificant. We interpret this as evidence that the behaviour of market makers, rather than individual investors, may be responsible for the relation between returns and weather.

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