When the El Niño rains start falling this weekend, so too will crime rates, or so says conventional wisdom.

The long-held belief is that bad weather keeps the criminals indoors, just like it does everyone else.

But that conclusion is not as accurate as some might think, according to criminologists and law enforcement officials. One police lieutenant warns that rain can make people more susceptible to some crimes. Among dozens of studies on the correlation between weather and crime, not a single one shows a relationship between rain and crime, according to criminologist Ellen G. Cohn of Florida International University, who analyzed the research.

But that doesn’t mean all hope for safer streets is lost. Though the rain won’t keep crime away, the cold might, explained Cohn, an expert in weather and crime.

“There has been a ton of research on weather and crime … and the only variable that consistently shows a relationship is temperature. Not rain, not snow, not humidity,” she said.

As temperatures rise, so does crime. But that is only to a point — if it gets too hot, potential criminals get more focused on escaping the heat than committing crimes, she said.

The inverse is true as well — colder temperatures equals less crime, though there is less research on the effects of colder temperatures.

Research indicates that the effect of temperatures is relative to the location. A 40-degree night in Minneapolis may not see much effect. But in Los Angeles, it could provide just the deterrent to keep crime at bay.

Pasadena Lt. Tracy Ibarra said colder weather creates less opportunity for certain types of crimes, but perhaps more opportunity for others.

“You don’t have as many people shopping or, let’s be honest, drinking at bars. … So then you have less fights, fewer calls to service in that area,” she said.

But that doesn’t mean police departments aren’t busy. There’s the obvious increase in traffic accidents when it rains, but the Pasadena Police Department also responds to a lot of burglary alarms during big storms, Ibarra said.

Many are false — the result of high winds and broken branches.

“But we aren’t the only ones who know that. … The false alarms can mask real burglaries. People who are committing burglaries take advantage of that, as well as there being fewer people around, fewer witnesses,” Ibarra said.

People also can be lazy about what they leave in their car if it’s pouring rain, tempting potential thieves.

“They think, I’ll get that bag or purse later and then never go back,” Ibarra said. “So we really don’t see a drop in burglaries when it rains.”

A look at local crime data in the cities of Los Angeles and Pasadena hardly reveals any rain-inspired crime trends either.

Southern California was struck hard by El Niño and record rainfall in the early months of 1998. In January, February and March of that year, crime, including homicides, aggravated assaults and burglaries, was indeed down in Los Angeles. But those rates dropped even further the following year and were part of a nearly decade-long drop, according to LAPD statistics.

The region also saw torrential rainfalls in January and February 2005.

Similarly, aggravated assaults and burglaries were down in Los Angeles that year, compared to 2004, but they continued to drop the following year, while homicides remained flat.

Pasadena crime rates remained relatively stable during the rainy months of 2005 compared to the year before and after, though assaults were up slightly.

The tricky thing with understanding weather and crime, Cohn says, is removing all other variables.

For example, crime goes up in the summer, but is that a result of more people being away from their homes on vacation, teenagers being out of school or temperature, Cohn asked.

Still, she said researchers have controlled for these other factors in concluding that more crime happens in higher temperatures.

Understanding the relationship between rain and crime can be even more challenging since it doesn’t rain all day long and police departments don’t track the exact weather when a particular crime occurred.

But Cohn said many police departments are paying closer attention to the link.

“There is a move toward incorporating this sort of thing into prediction models … so they can more accurately allocate workloads, better use resources,” she said. “It’s a way to be more efficient.”