There are two reasons for this:
Your nasal passages produce mucus to humidify the air before it reaches the lungs. Cold air can't hold as much water as warm air so it's drier, forcing an increase in mucus production.
Your nasal passages also heat the air for your lungs, particularly air breathed through the nose. To compensate for colder temperatures the blood vessels in this area dilate to increase blood flow. This also increases the activity of the mucus membranes.
During a normal day your body will produce about a liter of mucus, most of which is swallowed. The effects of cold, dry air can increase mucus production past the point your body can handle, allowing some of it to drip out of your nose.
This is not an indicator of a viral upper respiratory tract infection, commonly known as a "cold." However, during cold weather you are more likely to stay indoors, increasing your chance of contact with an infected person. Also, low humidity can increase the dispersion of viruses which can further your chances of contracting a cold.
Posted 4448 day ago