LOUISVILLE, Ky. (WDRB) -- It's tempting to leave your car running in the colder temperatures, but police say that's the exact time not to do it.

The puffs of smoke from a car's exhaust become much more visible and are a dead giveaway for thieves.

Police say when the temperature drops, the number of stolen cars goes up.

LMPD says leaving your car on with locked doors is not only unsafe, but it's illegal and you can get a ticket.

Police say thieves are looking for an opportunity, and they're young and fast.

"In the case where we're making arrests, we're seeing juveniles. Kids standing at the bus stop. Instead of waiting in the cold, [they] see a nice warm car and decide to take a ride in it for a little while," LMPD Second Division Detective Jason Grissom said.

Grissom says just in his division, he's recovered 17 stolen cars.

Copyright 2015 WDRB News. All rights reserved.