When you open the door to your house on a cold day, you feel a cold draft. What causes this?

Inside it's usually warm so when you open a door to outside which is cold a cold front and a warm front will be exposed to each other cold will win usually since cold goes below the warm front.

Explanation:

water at room temperature is flowing through a pipe with pressure taps located 10 ft apart along one section of the pipe, which are connected to a mercury (s=13.6) manometer.