Nobody knows for sure, but one reason is that the nose has to “work overtime,” when the inspired air is cold, said Dr. Ralph Metson, a sinus surgeon at the Massachusetts Eye and Ear Infimary.
The nose is a kind of “fancy air conditioner” whose job is to warm and humidify the air we inhale, Metson wrote in an email. When the air we breathe in is unusually cold, the nose kicks into high gear to warm and humidify it – blood vessels dilate, mucosal tissue swells and glands secrete extra mucus. This extra mucus manifests itself as a runny nose.
In addition, as the newly-heated, newly-moist air is exhaled, the moisture in it condenses when it hits the cooler, outside temperature, and then drips out as fluid, said Dr. Andrew Lane, director of the division of rhinology at the Johns Hopkins University School of Medicine.
For people who have a major problem with this kind of runny nose, technically called “vasomotor rhinitis,” there is a prescription nasal spray called ipratropium bromide, which may help damp down this reaction.