Motorists are being urged not to leave their vehicles unattended while running as they wait for them to defrost on chilly mornings.
One car has already been stolen this year in the county after keys were left in the ignition.
Insp Paul Lawrence said: “The weather has been pretty mild over the last month, but colder weather appears to be on the way. Defrosting the car in the mornings isn’t a pleasant task and we’ve seen in the past that leaving the car running whilst you go back inside where it’s nice and warm can give thieves any easy opportunity.
“Your insurance companies may not cover you if your vehicle is left insecure or the keys are left in the ignition. Those few extra minutes in the warm, could end up costing you thousands of pounds. Never leave your car unattended with the engine running.”
The frosty weather also makes driving conditions more hazardous so police have issued the following safety advice.
Ensure the windscreen and other windows are fully deiced and demisted before driving, make sure wiper blades are in good condition and there is plenty of washer fluid, slow down, avoid braking sharply where possible and make sure tyres are correctly inflated and in good condition.
Motorists can also help protect in-car items such as sat navs by registering them for free at www.immobilise.com, which helps police return recovered stolen items to owners and is a deterrent to criminals.
For more advice visit www.herts.police.uk/crimeprevention.