Why does breathing through the mouth dry the windpipe and cause dry sore throat?

Nose humidifies more: Your respiratory system is designed with a natural humidifying organ, your nose. When you do not breathe through it, the air bypasses its humidifier and reaches the throat and lungs still dry. When you do breathe through your nose, the air is 100% humidified by the time it reaches those places. Your nasal membranes secrete about a quart of fluid a day, much of it water vapor reabsorbed in the lung.