The strange and surprising ways the sun affects what we do.

The Tipping Point Sunshine makes us nicer, inducing us to want to help others. On sunny days, regardless of the temperature, we answer more survey questions from people with clipboards and tip more generously. In dark Atlantic City hotel rooms, people give higher tips if the bellhop tells them it's sunny outside than they do if he tells them it's cloudy. Nice days put us in a good mood, which engenders helping and generosity, explains David Strohmetz, a psychologist at Monmouth University. "When we're in a good mood, we want to maintain that mood." The Wisdom of Clouds The weather even affects college applications, helping determine which types of applicants are admitted. Applicants who are strong academically are more likely to be admitted on cloudy days, whereas candidates who are strong socially are more likely to be admitted on sunny ones. Cloudy days call to mind thoughts of staying inside to read or study, explains Uri Simonsohn, the behavioral economist at the University of California at San Diego who conducted the study. A previous study found that cloudy forecasts prime people to think about academics. Summer Stock Sunshine influences the stock market, which is three times more likely to go up when it's sunny in the city of the exchange. Investors feel happier on sunny days, but mistakenly attribute that happiness to stocks' prospects, explains David Hirshleifer, a finance professor at the University of California at Irvine. "It's a halo effect," says Hirshleifer. "A generalized optimism latches on." Season Tickets We buy more lottery tickets on cloudy days—not to boost our mood, but because weather-induced bad moods deplete self-control, making us more vulnerable to temptation. Darkness at Noon Suicides go up in warmer months. Seeing others frolic outside reinforces depressing feelings, says Michael Puniskis of Middlesex University. And sunshine may give depressed people the energy to finally take action. The Sin Also Rises On less sunny days, we compensate artificially, using more alcohol, coffee, tobacco, and chocolate to stimulate ourselves and elevate our mood. Bright Light, Big Cities How many hours of sunlight do American cities get per day? Phoenix: 10.3 Los Angeles: 8.8 Atlanta: 7.6 New York: 7.4 Chicago: 7.2 Seattle: 5.6 By Jay Dixit