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Name: Alice
Status: student
Age: 8
Location: N/A
Country: N/A
Date: 1999-2001

Why can you see your breath outside on a cold day?


Your breath has moisture (water vapor) in it. When you exhale into very cold outdoor air, the moisture-laden atmosphere from your lungs becomes chilled to the point where the water condenses into a fog.

Try this: Exhale onto a mirror that you stored in the refrigerator for an hour to get it nice and cold. Notice how the mirror fogs up from the condensation of the moisture in your breath. Notice also, that your breath between your mouth and the mirror made no foggy cloud in the air. That's because the air in the room wasn't cold enough to condense the moisture in your breath into a cloud. However, when the breath finally reached the mirror, condensation did occur. This little experiment demonstrates that you are exhaling moisture all the time. It's just that you don't see it when the air around you is warm.


Alice -

Warm air (your breath) can hold more water vapor than cold air. When you breath out, your breath cools and the water vapor must leave the air. It condenses and you see it as a cloud.

Larry Krengel

Cold air does not hold as much water as warm air. As warm, moist air leaves your mouth and cools, the water that was dissolved in it condenses into tiny droplets. The same thing happens on a larger scale when clouds form.

Tim Mooney

Seeing your breath when you're 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 can't be identified by the eye either, but we see the light reflected off them, much like smoke from a cigarette.

So we really don't 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.

You can see your breath on a cold day because the amount of water vapor in your warm exhaled breath exceeds the amount of water vapor that can held by the cold air. Any water exceeding that concentration will form water droplets which are very small so they look like the cloud or mist you see.

Vince Calder

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