Ask the doctor: Runny nose
Q. Why does the nose run in cold weather?
A. The inside surfaces of the nose are covered with blood-filled mucous membranes that warm and humidify the air as it passes by on its way to the lungs. When cold air hits them, they tend to secrete water and mucus. The colder and drier the air, the more water and mucus need to be produced and the more the nose runs. Taking a warm shower or bath can help a runny nose because air you're taking in is already moist and warm, so the membranes can relax and stop secreting water and mucus.
— Robert H. Shmerling, M.D.
Beth Israel Deaconess Medical Center