“It is so cold I can see my breath.”

How many times have you ever heard someone say this?

The relative humidity of the exhaled air is actually what matters. Conditions in the lungs are warm and extremely humid. When you breathe in air, conditions in your lungs add heat and moisture very suddenly.

Exhaled air is about 95 degrees with about 95% relative humidity. As air leaves your nose and mouth, it is quickly cooled by the ambient temperature.

If the outside air is either cold or very humid or some combination of both of these, the exhaled air will easily reach 100 percent relative humidity, causing some of the water vapor to condense into a cloud of visible water droplets. This is why we frequently "see our breath" when it is very cold, but also when the air is very humid, like in misty or foggy weather.