It was a Saturday in the heart of summer, the kind of day that averages more than two homicides. Yet the police reported no killings.
One other thing happened that day: It rained.
In fact, an analysis by The New York Times of rainfall and homicides for the last six years shows that when it rains substantially in the summertime, there are fewer homicides.
When there was no precipitation, there was an average of 17 homicides every 10 days. But when there was an inch or more of rain, the average dropped to 14.
That does not surprise Vernon J. Geberth, a former Bronx homicide squad commanding officer. He said that when there was a downpour, the police would sometimes joke, “The best cop in the world is on duty tonight.”
The gap is even wider when looking just at Saturdays in the summer. Those are the days that typically post the highest number of homicides in a year. When there was no rain, the average number of homicides for every 10 Saturdays in summer jumped to 24. For every 10 Saturdays doused with at least an inch of rain, the average number was 18.
With a few more than 200 homicides so far this year, the city is on pace for a low not seen since the early 1960s. The first few days of July promise more of the same damp weather, with a chance of rain every day.
The difference in homicides on rainy days is more pronounced in the summer than in other seasons. And when there was less rain, there were more killings. For instance, when the threshold is lowered to half an inch of rain or more, the average number of homicides every 10 days climbs to 15. And when there was less than half an inch of rain, the rate was the same as when there was no rain.
The Times’s analysis is based on comparing the city’s daily rainfall with homicide data obtained from the New York Police Department, covering 2003 to 2008.
Some criminologists caution against reading too much into the differences. Ellen G. Cohn, a professor at Florida International University, who has examined links between weather and crime for more than two decades, said the impact of rain on crime was “not much.” While she has not examined homicides in New York, she said rainfall tended not to strongly predict homicides in the United States.
Some studies found that other crimes, like aggravated assault, went down when there was rain. Professor Cohn said one reason might be that assaults more often involve strangers, and rain reduces the chances that people who do not know each other will encounter one another.
That theory is part of the reality on New York City streets, according to Mr. Geberth, who worked during the city’s crack epidemic in the 1980s. “In good weather, there are more people out on the stoops,” he said. “Somebody bad-eyeing somebody else, and the next thing you know, you have been dissed.”
But he believes the logic extends to homicides as well. “It doesn’t take much to get ‘deaded’ in certain neighborhoods. All you got to do is look sideways at the wrong people, and bingo, something gets set off and it’s crazy.”
He added, “On bad weather days, people are apt not to run into each other that are carrying grudges from the day before.”
Professor Cohn said that murders nationally were less likely to involve strangers. And rain does not deter domestic homicides, which usually happen indoors. Precipitation is also unlikely to stop killings that stem from illegal drug deals gone wrong.
Steven Messner, a criminology professor at the State University of New York at Albany who has studied homicides in New York City, agreed that rain was not likely to stop many killers. “People adjust to climate,” he said. “They get umbrellas, they go out. Humans are adaptable.”
Other factors may be in play. Rainfall usually leads to cooler weather, and cooler days tend to have fewer homicides, which may explain any drop in killings, Professor Messner said.
Rain can also make it harder for detectives to solve homicides. Evidence gets washed away.
“I remember standing out in the middle of a rainstorm with a body in the middle of the street, trying to work out what happened,” Mr. Geberth said. “Depending on how hard it is raining, we are losing stuff. We are losing bodily fluids. We are losing shell casings. That exchange of material from touch DNA to hair fibers is dissipated by the elements.”
In addition, witnesses can be harder to find in bad weather. Fewer people are out; those who are outside are usually distracted or are seeking shelter.
Edward C. McDonald, 42, who worked on homicides in the Bronx, said he could remember working on only one rainy-day homicide. That was near Yankee Stadium in 2005, when a gunman pulled up in a vehicle and shot a man sitting in a double-parked car on the Grand Concourse.
Even though it was a busy area with stores nearby, and the killing occurred just after a game, the police had a hard time finding witnesses.
“I was standing, guarding a crime scene and getting drenched,” Mr. McDonald said. “People were jumping in cabs. There were no witnesses, not because people saw it and didn’t want to talk, but everybody was concentrating so hard on the rain and getting out of there, nobody saw anything.
“Even stores across the street, even if those people saw anything, it would be hard for them to see with the torrential downpour,” he said.
“The other side of the coin was, if you came in on a nice summer day, you could see people picnicking in the parks, and we would say, ‘We are waiting for something to happen,’ ” Mr. McDonald said.
“Everybody’s out partying, people start drinking, old beefs pop up, and people get their beer muscles out and start fighting.”
And for those hoping a good rain will at least cut down on homicides, keep in mind: there are not that many summer days that get at least an inch of rain. It happens only about 1 in 20 days.It only seems to rain more.
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