What seems like a pandemic of mass shootings and gun violence in the US may confirm a tendency that police departments have long warned about: killings rise with warmer weather.

Criminologists have been writing about the correlation for decades, with a more recent study focusing on the precise relationship between temperature and crime rates.

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For those who have researched the issue, there are both common sense and maybe less evident mechanisms at work.

First, the obvious: "It's hard to shoot somebody if there's nobody around," David Hemenway, a health policy professor at Harvard TH Chan School of Public Health, told AFP, explaining why gun violence is lower in poor weather.

A second, more contentious theory is that heat may incite violence.

While there are several reasons for the rise in gun violence in the US, the weather may play an increasingly critical role in a world that is rapidly warmer owing to climate change.

Given misconceptions about the north-south difference between the US and Italy, as well as between northern European states of Scandinavia and southern Mediterranean countries, Hemenway said he has long been interested in the association between heat and increased crime.

In 2020, he co-authored a report in Injury Epidemiology with his then-graduate student Paul Reeping that looked at the city of Chicago between 2012 and 2016.

The Chicago Tribune reports were used to calculate the number of gunshots each day, which were then compared to the daily high temperature, humidity, wind speed, temperature difference from the historical average, and precipitation type and amount.

They discovered that a 10 degree Celsius increase in temperature was substantially related to 34% more shootings on weekdays and 42% more shootings on weekends or holidays.

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They also discovered that a temperature 10 degrees Celsius higher than the average was connected with a 33.8% greater likelihood of shootings.

"In the winter, there were more shootings on those days which wouldn't have been hot in the summer but were warm for winter," stated Hemenway.

Another recent study, headed by Drexel University's Leah Schinasi and published in the Journal of Urban Health in 2017, looked at violent crime in Philadelphia.

Schinasi told AFP that she was interested to see if her observation of people being crankier on hot days meant higher rates of crime.

She and co-author Ghassan Hamra discovered that violent crimes occurred more often in the warmer months of May through September, with the greatest rates on the warmest days.

The discrepancy was especially noticeable on pleasant days throughout the colder months (October through April) versus colder days during those months.

When temperatures hit 21 degrees Celsius (70 degrees Fahrenheit) throughout that time period, daily rates of violent crime were 16% higher than when temperatures averaged 6 degrees Celsius (43 degrees Fahrenheit) over those months.

Hemenway argues that both of the primary ideas on the issue, that more people being outside increases the possibility of violent interactions and that heat makes individuals more aggressive, might be correct.

Research published in 2019 by the National Bureau of Economic Research includes placing university students in Kenya and California in either hot or cold rooms and assessing the influence on a variety of behavioral categories.

It found that "heat significantly affects individuals' willingness to voluntarily destroy other participants' assets" in the form of gift cards and vouchers.

Hemenway admitted that there are considerably more important factors of gun violence than temperature.

These include the fact that there will be 393 million weapons in circulation in the United States by 2020, more than the number of people, and that many states have moved in recent years to relax rather than tighten prohibitions.

A better understanding of the association with the weather, on the other hand, might have policy ramifications, such as providing additional activities for young boys to keep them off street corners on the warmest summer days, and increasing police presence in crucial places based on predictions.

Hemenway believes it to be "harm reduction", noting that "even if this wasn't a gun problem, I suspect we would find the same thing if we had evidence about fights and assaults. What the guns do is make hostile interactions more deadly."

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