Isothermic saturation boundary: point where gas hits body temperature and 100% relative humidity. Usually at secondary bronchi.
During inspiration, cool, dry gas flows past warm musosa. Fluid evaporates from the mucosa into the dry gas. At the same time, heat is transferred by convection from musosa to gas, cooling the mucosa.
During expiration, warm, humid air flows past the cooled mucosa. Gas condenses on the cooler mucosal surface, and heat is transferred from gas back to mucosa. The warmer the environment, the less the gas can be cooled as it is exhaled, and the more fluid is lost.
This process relies on turbulent flow, which is promoted by the nasal turbinates.