WASHINGTON: From the Texas school massacre to a Tulsa hospital shooting and many less-reported incidents, a recent spate of gun violence across America bears out a trend police departments have long sworn by: murders go up in warmer weather.

The link has been written about for decades by criminologists, with more recent research drilling down on the precise relationship between temperature and crime rates. For those who have studied the question, there are common sense as well as potentially less obvious mechanisms at play.

First, the more obvious: “It’s hard to shoot somebody if there’s nobody around,” David Hemenway, a professor of health policy at the Harvard TH Chan School of Public Health, said, explaining why gun crime is lower in bad weather.

A second, more controversial idea is that heat itself — as opposed to weather that encourages people to be out — might rev up conflict.

While there are many causes behind the rising tide of gun violence in the United States, weather could play an increasingly important role in world that is fast warming due to climate change.

Hemenway said he had long been interested in the relationship between heat and higher crime given stereotypes about the north-south divide within the United States and Italy, as well as between the northern European states of Scandinavia and southern Mediterranean countries.

In 2020, he co-wrote a paper in Injury Epidemiology led by his then-graduate student Paul Reeping examining the city of Chicago between 2012 and 2016.

The paper used reports from the Chicago Tribune to get the number of shootings per day, and then matched those against daily high temperature, humidity, wind speed, difference in temperature from historical average, and precipitation type and amount.

They found a 10 degree Celsius higher temperature was significantly associated with 34 percent more shootings on weekdays, and 42 percent more shootings on weekends or holidays. They also found a 10C higher than average temperature was associated with 33.8 percent higher rate of shootings.

In other words, said Hemenway, it’s not just heat that’s important, but relative heat: “In the winter, there were more shootings on those days which wouldn’t have been hot in the summer but were warm for winter.” Another recent paper, led by Leah Schinasi of Drexel University and published in the Journal of Urban Health in 2017, looked at violent crime in Philadelphia.

Published in Dawn, June 5th, 2022