In a paper on weather and stock market returns, the researchers write: There is strong correlation between sunlight exposure and behavior. Among other things, lack of sunlight exposure is linked to depression and suicide. We study how market returns are related to the daily level of clouds and the duration over European cities from 1980 to 1992 and we find a strong negative relationship. We suggest traders to exploit the return patterns by trading every day based on our results. b) Comment on the results of this article by referring to the market efficiency and anomaly literature