The nose is a complex organ, meant not only to help us smell wonderful (or not so wonderful) smells but also to act as a filtration device for incoming air. It might be surprising to learn that your nose secretes, when you’re healthy, about 32 ounces (0.94 liters) of mucus every day. This doesn’t tend to make your nose run; instead most of this mucus falls to the back of the throat and is eventually swallowed. These secretions help to humidify, warm up, and filter incoming air, in conjunction with the small hairs in the nose.
So why does a nose run when it is cold? This is an excellent question, which fortunately has a fairly simple explanation. Underneath the glands that secrete mucus, you have huge amounts of tiny blood vessels, which help supply these glands. In cold weather, these blood vessels dilate or grow larger. This means you have more blood supply to your nose, which in part protects your nose from the cold, but it also means your nose will begin producing greater amounts of mucus and liquid.
Additionally, you will notice more mucus when it is cold because the nose has to work overtime to warm up air that is inhaled, which is coming in at much lower temperatures than normal. You might even notice a runny nose when it is cold only by a few degrees. Temperatures just a few degrees below room temperature can make the nose run.
There’s also the issue as mentioned above of normal mucus and secretion production. At room temperature, your nose is already producing 4 cups (0.94 liters) of fluid and mucus a day. Only so much of it can fall to the back of the throat and be swallowed. When mucus and secretion production increases to warm the air, you will see the nose run because you have excess secretions. In other words, some of it has to drip out the front because you have an excess supply. When you get into a warmer room a few moments later, you won’t notice your nose continuing to run because it has warmed up and the blood vessels become more constricted.
Observing a runny nose in the cold has led many to believe that cold weather causes illness. This isn’t actually the case, and the nose helps through filtration to try to avoid viruses. Of course it doesn’t avoid them all. Being out in cold weather for a few minutes may actually help decrease congestion a little since mucus will be naturally released through the front of your nose, allowing you to have a few good blows. Usually, you’re just as stuffed up again once you’ve gone back indoors.