Photo: You Know It's 'Chicago Cold' When ...

CHICAGO (CBS) -- You know it's cold when the inside of your doors and windows get frosty.

This happens when the extremely cold air seeps in and mixes with the humidity inside the room, creating a frosty glaze.

The photograph above comes from a Reddit user, who posted a picture of a frosty interior doorknob.

Chicagoans are also seeing frost on their windows and door frames.