People also become more likely to retaliate. That can turn into a cycle, he said, and it doesn't take much to see a big effect on crime statistics.

He helped conduct a 2010 study that showed that as temperatures rise, so do people's tempers. The research examined the impact climate change could have on violent crime.

Using data from 1950 to 2008, the professors predicted that if the country's annual average temperature were to increase by 8 degrees, the number of murders and assaults would jump by 34 per 100,000 people.

Others have said the relationship between crime and temperature is complex.

James Alan Fox, a professor of criminology at Northeastern University, studied daily crime counts and temperatures in Columbus, Ohio, for a year. He discovered crime did increase with the temperature but only to a point. Once it got too hot, crime reports dropped again.

He wrote in 2010 that crime was highest when temperatures were hovering in the mid-80s, but petered out as temperatures climbed into the 90s.

He also found temperature had the greatest impact on crimes outside of the home.

Most of the overnight incidents in Baltimore took place outside, Guglielmi said.

But some research has raised questions about the link between heat and murder in the city.

A 1995 article published in the Journal of Quantitative Criminology looked at crime in Baltimore over a period of eight years to see whether temperature had a direct influence on the number of killings. The study found that "homicide in Baltimore is a surprisingly consistent — or constant — process, showing little or no variation on many temporal factors important at the national level."

On Thursday, in a home in the 800 block of Arnold Court in the Gay Street neighborhood, two women sat on a back porch speculating what may have prompted a shooting in their complex. They blamed the drug dealers who seem to proliferate in their alleys when it's warm.

"It's not even summertime and they're already outside, selling their damn weed and all that mess," said a woman who declined to give her name because the shooter has not been caught. "They stand out in front of your doors. You ask them to leave, they give you mouth."

The woman said she saw paramedics rolling the wounded man in Wednesday's shooting into an ambulance. All the while, she said, the man kept saying, "Please, just give me some cold water."

The woman looked at her friend and said, "Bullets are hot, I believe."

Baltimore Sun reporters Scott Dance, Kevin Rector and Carrie Wells contributed to this article.

Police urge anyone with information about the crimes to call 911 or 1-866-LOCKUP.

A violent night in Baltimore

Eight people were shot in six nonfatal incidents in Baltimore on Wednesday night and early Thursday morning, and all were expected to survive.

•The first reported shooting occurred about 6:30 p.m. in the 800 block of Arnold Court, close to the Kennedy Krieger Institute in East Baltimore. In that case, the man was shot multiple times.

•The second incident was in the 2500 block of Loyola Northway in the Greenspring neighborhood at 8:51 p.m., where police said two people were shot: a 20-year-old man in the left foot and a 19-year-old in the chest. Police transported the man shot in the foot to an area hospital, while the other man ran from the scene and walked into a hospital.

•In the third shooting, just after 11 p.m., a 19-year-old man was shot in the right shoulder in the 2600 block of Norland Road and ran to the 2100 block of Patapsco Ave. in the city's Lakeland neighborhood.

•A fourth shooting occurred about the same time when a 20-year-old was standing in the 1100 block of E. Madison St. He was shot multiple times.

•Police reported a fifth shooting about 11:57 p.m., a 24-year-old woman was shot in the 3200 block of Vickers Road in the city's Hanlon-Longwood neighborhood. A 28-year-old man was also shot in the incident, and police said he ran from the scene and walked into a hospital.

•In the sixth shooting, a 29-year-old man was shot in a vehicle at about 2:12 a.m. at the intersection of North Stricker and West Fayette streets, in the city's Franklin Square neighborhood. He was wounded in the left side of his body, police said.