Breathing - Just a Lot of Hot Air
Most recent answer: 10/22/2007
Q:
Why is it that when we exhale slowly, the air feels hotter than when we blow out intentionally?
- Renee Carter (age 19)
University of Akron, Akron, Ohio, USA
- Renee Carter (age 19)
University of Akron, Akron, Ohio, USA
A:
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 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
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 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)