Sunshine is an essential component in the growth of most living things and it now turns out that it can also have an impact on the stock market. A research paper entitled "Good Day Sunshine: Stock Returns and the Weather"
by David Hirshleifer from The Ohio State University and Tyler Shumway of the University of Michigan found that the difference between market returns on a sunny day versus an overcast day to be 0.09%, an annualized difference of 24.8%
Several psychological studies have shown that sunlight positively affects mood. People in a good mood are more likely to have an optimistic outlook on the future which, in the case of trading, translates into a more bullish sentiment. Past research has also indicated that people in good moods are not as likely to critically investigate evidence which may cause traders on sunny days to place less evidence on negative market indicators and amplify positive ones.
This research doesn’t mean you should change the channel from CNBC to the Weather Channel though. While the difference between sunny days and overcast days is quite dramatic, it does not include the transaction costs and commissions that would reduce any overall gain.