How can you blow hot and cold?

26 February 2012

Question

Hi Chris
My ten year old son Joshua Woolf asked me why the air that you breathe out when you blow out candles is cold as opposed to the arm air that you breathe out when you want to mist a window. I explained that the warm air comes from inside your body and the colder air is air that is being used from in front of your mouth but I'm not sure that this is correct. If I breathe in through my nose and hold it and then breathe out my mouth that air is cold or hot depending on how I breathe it out. Please clarify for us.
Thanks
Shelley Woolf

Answer

Dave - This is a really interesting one. We did this as a Kitchen Science a couple of years ago. If you blow with very pursed lips very quickly, it feels cold and then if you blow with an open mouth slowly on your hand, it feels warm. This is all to do with what's happening with the the air. If you're blowing through very narrow pursed lips, the air is going very quickly and it tends to mix in very strongly with the air around it. So, what's hitting your hand is mostly air from the room, it's moving quite quickly and quick moving air tends to feel cold because it moves heat away from your hand quicker. So mostly, what's hitting your hand is air from the room. Whereas if you breathe slowly with your mouth open, that's a much smoother jet of air, so it's a much wider jet of air, it doesn't mix in nearly as quickly. So what hits your hand is mostly air from your lungs which is warm.

You can actually get the warm effect from the fast moving air by putting your finger right up close to your mouth when you blow, it still feels quite warm and it also gets quite damp. That's because it doesn't have time to mix in with the air around it, so it's still warm and you can feel it.

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