FORT WORTH -- When the mercury rises, tempers flare.
Drivers honk more. Spouses argue. Strangers can be downright rude.
"There is a connection between heat and violence," said Wade Rowatt, an associate professor of psychology and neuroscience at Baylor University. "When temperatures are hotter, people are more irritated and hostile and likely to be aggressive."
So on Thursday, if North Texans seemed a little nicer, a bit more polite, there was good reason: For the first time in 40 days, temperatures did not reach 100 degrees -- and some areas even saw a little rain.
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But, officials say, those sunny dispositions will likely be gone today, with temperatures expected to surge back into the triple digits.
"Stay hydrated, stay inside and take frequent breaks," Rowatt said. "If people feel themselves starting to get angry, take deep breaths."
Maj. Paul Henderson, a spokesman with the Fort Worth Police Department, said assaults and domestic violence spike when it's hot, which is why police try to get in front of the problem by putting more resources in high-crime areas during the summer. And while overall crime is down compared with last summer, Henderson said, aggravated assault and domestic offenses are up, which he attributes, in part, to the excessive heat.
"People tend to be more agitated and irritated with this amount of heat," he said. "We are praying for in a break in the weather pattern."
So is Mary Lee Hafley, CEO of SafeHaven, Tarrant County's largest family violence shelter, which took in 14 families last weekend -- unprecedented for a two-day period. She believes that the scorching summer has something to do with an uptick in domestic violence victims.
"When you get overheated, you get grumpy and your ability to filter your behavior is reduced," Hafley said, "so oftentimes -- while it does not cause domestic violence -- it may lead to more violent episodes or more frequent episodes."
J.C. Barnes, an assistant professor of criminology in the School of Economic, Political and Policy Sciences at the University of Texas at Dallas, said that while research shows that aggravated assaults increase with the heat, the reasons aren't clear.
"One of the more common theories is that people just get agitated and, if you're confined to a small air-conditioned space, you can't get away from the person you are mad at," he said.
Rowatt pointed to several experiments that have documented the relationship between heat and aggressive behavior, including:
Students who took a survey in a room above 90 degrees reported feeling more tired, aggressive, and hostile toward a stranger.
Phoenix drivers whose vehicles weren't air-conditioned were more likely to honk at a stalled car.
Major-league baseball players are 60 percent more likely to be hit by a pitch when it's 90 degrees or hotter than when it's 80 or lower.
"There is a connection between heat and violent-crime rates, but the connection is correlation and not causal," Rowatt said. "People could be more irritable or anger-prone in hot weather, but more people go outside when it is warm."
And while Rowatt and Barnes rely on research and experiments to show how heat can affect behavior, one local family law attorney believes that she is seeing it firsthand.
Divorces are up this summer, said Elizabeth Parmer, whose office is in downtown Fort Worth.
"I can't speak for other lawyers, but I have seen an increase in business," Parmer said. "It started about the 20th day we had temperatures over 100. I can't correlate it directly, but I find it interesting."
Parmer said the past few weeks are reminiscent of Christmastime, when she sees more people seeking a divorce.
"Things come to a head at Christmas or the holidays -- people are under stress, and a lot of what has been brewing comes to the surface," she said. "I have seen a similar business bump this summer. I don't see it every summer. I literally have had some clients, who had previously been in for a consultation and decided they were going to tough it, say, 'I can't do it anymore.'
"People are grumpier."