At least nine people were killed in Chicago last weekend on some of the year’s warmest days and nights, says the Chicago Tribune. The toll could increase because at least three others are in critical condition. The rash of violence fits a pattern that plagues Chicago and cities across the country each year: As the temperature rises, so does the murder count. Since 1991, 56 percent of Chicago murders occurred from May to October, with the highest numbers in the hottest months, July and August. “There’s always a correlation between homicides and ice cream trucks,” said Andrew Papachristos of the National Gang Crime Research Center. “It’s a simple mathematical idea: If you have more people interacting, you have more chances of something happening.”
There have been 272 murders so far this year–83 fewer than this time last year. Studies have confirmed that crime increases with the temperature, said criminologist Ellen Cohn of Florida International University. She found that crime plateaus when the temperatures hit the middle 80s, and then it begins to drop again.