Why does your nose run in the cold?

One job of the nose is to condition incoming air, warming it towards body temperature and adding moisture before it reaches the delicate lungs. When you breathe cold, dry air, nerves in the nose trigger glands and blood vessels to pump out more watery fluid, a harmless reaction doctors call cold-induced rhinitis or skier nose. At the same time, warm, humid air you exhale cools as it passes through the cold nostrils and condenses into water, just as breath fogs a window. The result is the familiar drip that has nothing to do with an infection. The body produces well over a litre of mucus and nasal fluid every day, but most of it is quietly swallowed.