It happens every winter. You feel perfectly healthy indoors, then you head out to shovel the driveway or walk the dog, and within minutes your nose is dripping. You're not catching a cold, so where is all that liquid coming from?
There are two reasons working together. First, cold air dries out the lining of your nose, so your nose makes extra mucus to keep it moist. Second, when you breathe out, the water vapor in your warm breath condenses as it hits the cold air near your nostrils. That extra water builds up and spills out.
Your Nose Is a Built-In Air Conditioner
Your nose does a lot more than smell. One of its main jobs is to prepare the air you breathe before it reaches your lungs. It warms the air to body temperature, adds moisture, and filters out dust and other particles.
Inside your nose are curled ridges of thin bone called nasal conchae, also known as turbinates. They're covered in a moist lining with a rich blood supply, and their curled shape adds extra surface area. As air passes over them, it gets warmed and humidified. On the way out, these same surfaces pull some heat and moisture back out of your breath, which helps keep your lungs from drying out.
Why Cold Air Means More Mucus
To warm and moisten the air, your nasal passages need to stay coated in a layer of liquid mucus. Cold winter air tends to dry that lining out. So the mucous membranes have to work harder, producing more mucus to keep everything covered.
That's the first half of the drip. Your nose is simply making more of its protective coating than usual, and some of it has nowhere to go but out. That coating earns its keep, too: it traps dust and other particles before they can reach your lungs.
Your Breath Adds the Rest
The second half comes from physics. Your breath is warm and full of water vapor. When you breathe out into cold air, that vapor cools right at the edge of your nostrils and turns back into liquid, the same way a cold glass gets wet on a humid day.
You can see the same effect when your breath makes a little cloud on a frosty morning. Inside your nose, that condensed water adds to the mucus already there, and the combination ends up running down your upper lip.
Other Reasons Your Nose Runs
Cold weather is just one trigger. Allergies, colds, and the flu can all cause a runny nose, as can dry air and tobacco smoke. During a cold or flu, the lining of your nose produces extra mucus, which helps keep the infection from spreading deeper into your airways.
Crying does it too. When you produce lots of tears, the extra liquid drains through a small duct at the inner corner of each eye and right into your nose. That's why a good cry so often needs a tissue.
Most runny noses clear up on their own without any treatment. But if yours lasts more than 10 days, or you're worried about it, talk to your doctor.
So the next time you're sniffling on a winter walk, give your nose a little credit. It's working hard to keep your lungs warm and comfortable, even if it makes a bit of a mess doing it.
