It's another consequence of climate change.
Statistical analysis of dozens of studies of violent crime rates, wars, and empire collapses has led scientists at the University of California at Berkeley to conclude that rising temperatures lead to rising tempers.
"What we do is we follow populations over time and watch how when local conditions become warmer and cooler the likelihood of conflict goes up and down," says Solomon Hsiang, a professor of public policy at UC Berkley.
That means the hotter it gets, the more likely people are to lose it.
"That whole or small-scale personal level conflict, things like domestic violence, assaults, rape and murder. And it's also true for group level conflict. Those are things where there's two groups fighting with each other, it could be inner-ethnic violence, it could be civil conflict."
So he's saying climate change can lead to more wars. But is there a way to predict how violent we tend to get? Yes there is. In fact, he has developed a formula.
"After looking at a large amount of data we find that on average if temperature, for example in a United States county, were to increase by five degrees Fahrenheit we would estimate that the rate of personal violence would go up by four percent.
Citing statistics provided by the King County Sheriff's office in 2012, no surprise -- aggravated assaults go up as the weather gets warmer. There were 95 assaults in the first three months of the year while there were 137 in the next three months and 141 in July, August and September.
So if personal violence goes up 4 percent for every five-degree jump in the temperature, you really want to avoid strangers on hot days.
KIRO Radio's Owen Murphy contributed to this report.