The nose is built for heat exchange
Inside the nose, air does not travel through a smooth, straight tube. It swirls around curled shelves of bone called turbinates, which create a large contact area within a compact space. Those surfaces are covered by moist tissue rich in blood vessels. Warm blood continually brings heat close to the passing air, and the thin, damp lining transfers some of that heat as air moves toward the throat.
The result depends on contact: air that brushes more of the lining has more opportunity to warm. The nose also adds moisture, which helps keep the airways from drying out. These jobs happen together, so breathing through the nose conditions air before it reaches the lower respiratory tract; the mouth largely bypasses this first stage.
A useful buffer, not a perfect heater
The nose cannot make every breath reach body temperature, especially when air is very cold or breathing is rapid. But warming the air along the way reduces the temperature swing faced by tissues farther down the airway. The moist lining also loses water as it humidifies incoming air, which is one reason cold, dry conditions can leave the nose feeling irritated.
That exchange takes place in the first few centimeters of the respiratory system: a narrow, folded passage turns a cold breath into air that is warmer and wetter before it goes deeper.
