The heat is back, and so are concerns about violent crime in the region's urban neighborhoods.
From the minute it arrives officially at 7:09 p.m. Wednesday, summer will be cooking - flaring up with record-threatening heat the next few days, maybe making a run at 100 on Thursday.
Summer may be vacation time for most people, but for police in high-crime areas, it's the hot time.
Research in recent years has affirmed that the connection between heat and violent crime is more than lore. Even screening out other factors such as economic conditions, age, and education levels, heat in itself has been shown to be a significant contributor to explosive behavior.
In the tinderbox neighborhoods of Philadelphia, Camden, and Chester, where the summer sun soaks into the streets and sidewalks - and the human body - intense heat can be the match that lights the fuse.
At least one study warns the trend toward hotter summers could brew trouble in the nation's urban areas.
"The discomfort caused by the heat is the best explanation for summer increases in violence," said Craig A. Anderson, a psychologist at Iowa State University.
The correlation between increasing temperatures and crimes of aggression is remarkably clear, said Scott Sheridan, a Kent State University researcher who led a major study of crime in Cleveland. "You end up with an almost perfect line," he said.
Based on an Inquirer analysis of national FBI Uniform Crime Statistics, summer is the busiest season for violent crimes, with rates almost 20 percent higher than in winter.
All the urban rioting of the 1960s, including in Philadelphia, occurred in summer.
Nationwide in 2004 (the most recent year for which statistics are available), 547 more people were slain in August than in February - 1,554 compared with 1,107. As for aggravated assaults, July's total exceeded February's by more than 20,000 - 80,362 compared with 59,844.
What is it about extreme heat that can induce violent behavior in some human beings, while leading others to explore new dimensions of lethargy?
The body is a marvel of thermoregulatory engineering. As heat increases, the hypothalamus, a tiny region of the brain that regulates body temperature, increases the rate of sweating. Sweat droplets give off a cooling effect as they evaporate. Think of the way the air cools in advance of a thunderstorm as water droplets evaporate in drier air.
When it's too hot or muggy, sweat can't vaporize. It simply adds to the discomfort, making a mockery of the body's hard work.
"As temperatures go up, you'll get more irritable," said Kenneth J. Neuburger of Thomas Jefferson University's emergency-medicine department.
How heat affects the mind isn't as well understood. But once the body temperature reaches about 101, "your brain function decreases," said Neuburger. And, yes, you might make decisions you regret.
One study documented that major-league pitchers are more likely to hit batters with pitches on hot days.
Scientific inquiries into the heat-violence relationship date to the 1700s, Anderson said. Some of the earlier laboratory studies involved the use of kerosene heaters.
"The heat hypothesis has been repeatedly confirmed," Anderson wrote in a paper published last year.
In Upper Darby Township, on the border with West Philadelphia, Police Superintendent Michael Chitwood said he didn't need research to know that summer can bring trouble. He's ready, he said, thanks in part to $25,000 in federal grant money.
The department already has executed several season-opening drug busts. "We usually start off the summer with a drug bang," he said.
With school out for the summer, he has beefed up bicycle patrols, and is deploying more officers in problem areas, such as those bordering the city and around the Second Ward Playground.
He also is relying on Nashid Furaha-Ali, the civilian who is the township's police liaison officer and staffs an office in one of Upper Darby's toughest neighborhoods, to help keep a lid on trouble.
In Chester, Mayor John Linder said he did not want to be too specific about hot-weather plans, but he did say that state and county police, along with U.S. marshals, would be available if needed in the heat.
"We won't hesitate to bring in other help," he said. "We have to show a different face.
"We're going to make it hotter and harder for these guys."
Contact Anthony R. Wood at 610-761-8423 or firstname.lastname@example.org.