Actually, according to Today's Health, there are a couple of reasons. The mucus normally secreted in the nasal passages collects on the ends of the cilia... fine hair like structures that move the mucuus to the throat to be swallowed rather than into the lungs. (One reason why a sore throat of precedes the onset of a cold). When it's cold outside, the normal mucus secretions collect on the end of the cilia but the silia are stiffened from the cold air. This cause the mucousa to then drip downward thorugh the nose instead of being routed to the throat. Secondly, the other cause is condensation. "When the cold outside air mixes with the warm air in our nose it condenses. The opposite also happens if we've been outside for awhile and come back in to heated air".