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Question:
Seeing your breath when you are outside lets you know that it is indeed cold..! The reason you can "see" your breath is due to the water vapor in your breath. When you breathe in, water vapor is added to the air by your lungs, along with the carbon dioxide from your body. When your breath leaves your warm body, and comes in contact with the cold air, it is immediately cooled. As it cools, the water vapor, which you cannot see, condenses into tiny water droplets, very much like the droplets in a cloud or fog. The particles are so small that they cannot be identified by the eye either, but we see the light reflected off them, much like smoke from a cigarette. So we really do not see our "breath" at all, but we see the condensed water vapor droplets in our breath. As soon as the droplets form, they continue to mix with the outside air, and are quickly evaporated in the drier air, and your "breath" disappears. Wendell Bechtold, meteorologist Forecaster, National Weather Service Weather Forecast Office, St. Louis, MO Does your breath fog more or less if there is less humidity in the cold air? it would seem that if what we see is condensed water vapor in our breath then the less water vapor in the outside air, the more our condensation would show?


Replies:
Actually, the opposite is true. Humid air conducts heat away from the breath more efficiently causing it to condense faster and in greater volume. Because high humidity slows the evaporation, the "breath" persists longer.

R. W. "Bob" Avakian
Instructor
Oklahoma State Univ. Inst. of Technology


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