However, if you have to use a door key, I would not lock the door when it is cold enough to freeze rain. Or if it is below 10 degrees. The freezing rain is pretty straight forward. If you can't get your key past the chucks of ice on the outside into the the lock itself, it is going to be difficult to unlock the door. Once you get down to 10 degrees or below things change even on the inside. You unlock your car with no issues in the morning and drive to the mall that is 25 minutes away. On the drive there you run the heat as anyone would do in this kind of weather. You park the car and with out thinking anymore about it, you lock your doors and pop into the mall. After shopping for an hour or two, you decide its time leave before you spend all your Christmas money. You make your way back to your car to find yourself unable to get the key to go into the lock. Now your calling for someone to come and get you .... bummer.
What Happened? On the drive to the mall all the moisture inside the car was heated and allowed to move about freely. Once the car was parked it didn't take long in the 10 degree or below weather to freeze up, including your locks. So, carry some lock deicer on you in the winter or maybe just leave the door unlocked.