Ask A Scientist© General Science Archive

name        Alice
status      student
age         8
Question -  Why can you see your breath outside on a cold day?

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Alice,

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.

Regards,
ProfHoff
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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
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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
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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.
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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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