Correlation between weather and homicide rates unclear

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As temperatures dipped in February and March, Baltimore's homicide count dropped, as well, prompting many on social media to wonder if there's a correlation. Arguments on both sides can be bolstered by just looking at the first three months of 2014.

Temperatures over that period were on average the seventh-coldest on record at Baltimore-Washington International Thurgood Marshall Airport. Nearly a foot of precipitation was measured — about 2 inches more than normal.


In February, 10 people were killed in Baltimore, seven in March. The last time Baltimore saw so few people killed in one month was June 1983.

While February and March's low homicide totals seem to correspond with low temperatures, in January — also a cold month — 27 people were killed.


That's 14 homicides over the average for the month during the past four years.

While many crimes show seasonal trends, Gary LaFree, a criminology and criminal justice professor at the University of Maryland, said cold weather does not have as much of an impact on homicides because many are domestic-related. That can cause monthly homicide totals to tick upward in winter because so many people live in close confines.

Comparing weather data with historic monthly homicide figures shows no clear correlation.

For instance, in January 2004, the 11-th coldest January on record, 23 people were killed in Baltimore. That's nearly five homicides above average for the month when comparing the last 44 years.

What was the temperature in June 1983, the last time Baltimore saw just seven homicides in one month? Weather data recorded at BWI shows the average temperature for the month was 72.1 degrees.

A decade later, when a similar average of 72.2 degrees was recorded in Baltimore for June 1993, 33 people were killed.

In August 1990, 42 people were killed in Baltimore. So it must have been one of the hottest months ever, right? In fact, it was the 89th-warmest month out of more than 140 years recorded, according to weather data.

Does snowfall have an impact? Four years ago, the blizzard commonly known as "Snowmageddon" paralyzed the Mid-Atlantic region in February. That year, between Feb. 1 and March 23, city crime data shows that 71 nonfatal shootings and homicides combined occurred in Baltimore.


Each of the next three years saw lower combined nonfatal shootings and homicides during the same period.

The same weather-crime questions are often debated in Chicago, a city that is experiencing a sharp decrease in violence.

Late last month, Chicago police said all shooting incidents, including gun killings, were down 30 percent in the first two and a half months of the year, compared to the same period in 2013. Homicides also decreased, from 58 to 49.

Yet the winter has been Chicago's third-worst in terms of freezing temperatures and snow accumulation.

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Chicago police Superintendent Garry McCarthy said the drop in violence had more to do with his police force's gang-monitoring operations over the past year and a half than the winter weather.

"Well, it hasn't been winter for the last year and a half," he said. "It's not an exact science. Sometimes you can reduce shootings and the murder rate can go up."


Baltimore Sun reportersQuinn Kelley and Scott Dance and the Chicago Tribune contributed to this article.