system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does being out in the cold make your nose run?
gpt
Cold, dry air can damage the mucous membranes in your nose as well as the bronchioles (the thousands of delicate little sacs that fill with air when you breathe!) in your lungs. So to prevent this damage, your mucous membranes make extra mucous that both warms and moisturizes the air you breathe in through your nose. This extra mucous is what is running and dripping in/from your nose.

Edit: Condensation is also a secondary contributing factor to your runny nose. When cold air hits warm air, the reaction produces moisture (essentially water). That condensation mixes with the hyper-drive mucous being made and adds to your flow/drainage.