Well let's begin at the beginning. No matter how cold and dry that air is, your nose has to make sure the air is heated to 98.6 degrees and is 100 percent humidified by the time it completes the 8-inch journey to your lungs. If the air isn't properly moist and warm, it will injure lung tissue. Proper humidification depends mostly on glands in the lining of your nasal and sinus cavities. Each day, those glands crank out about 2 quarts of fluid to lubricate the mucous membranes in your sinuses, nose, mouth, and throat.
Cilia are threadlike cells that wave back and forth over some of the surface tissue.
They help keep the nasal passageways clear of particulate matter. From time to time, and especially in the winter, the mucus dries out. That slows down the cilia. A virus can stop the cilia altogether. When the cilia stop waving, secretions pool in the back of the nose.