Renee -
I assume that what you're referring to is that if you
breath out slowly or quickly onto your hand, the air either feels
warmer or cooler on your skin. Actually, the air in either case is
nearly the same temperature. The difference is only in how your skin
feels it.
I want to be a little careful answering this, because
I just tried the same experiment and didn't get the same results. The
main difference I noticed was that when I blew out slowly, my hand
stayed warm longer. That's just because the warm air from my breath was
on my hand longer. The air is warm because it's been in my lungs, where
it reaches about the same temperature as the rest of the inside of your
body.
In general, there can be two main reasons why air at some
temperature feels warmer or cooler depending on how fast its blowing
past you. One is that, if the air moves really quickly past, there's
not enough time to form a little layer close to body temperature near
your skin. If the air is cooler than you, that means that the quickly
flowing air will take heat away from you more quickly. Of course that
isn't true when the air is already hotter than you or about your
temperature, like your breath.
This is the same reason that we
have windchill. On cool days, it will feel colder if there's a lot of
wind. This is because the fast-moving wind actually carries the warmth
away from your body. For a cool website all about windchill, check out
The Windchill Homepage.
The
other effect comes from evaporating moisture on your skin. If dry air
blows past you, moisture from your skin evaporates into it. That cools
you (see link to evaporative cooling). So even on hot days, if the air
is dry, wind can make you feel cooler. The air from your lungs is
already very moist, however, so this effect doesn't much change how
your breath feels.
-Tamara & Mike
(published on 10/22/2007)
