Why does your breath get colder when your mouth hole is smaller?

On “In Our Time“, Radio 4, on Thursday 16th May 2013, about 17 minutes in, we were reminded of the old controversy of people changing the temperature of their breath at will, as for example in “the Aesop’s fable“? Actually, most of us can – sort of:

We can warm our hands when they’re cold by blowing slowly with our mouths fairly wide – hair-drier style. In that case, our breath warms our hands, as its nearer our core temperature than our hands, particularly on a cold day.

However, when blowing hot soup, the soup is obviously much hotter, so it’s cooled by our breath, even if the breath is the same temperature as before.

But less obvious is when you start to blow on your hand with your mouth fairly wide open, and during the same breath, purse your lips and make your mouth hole smaller. Your hand feels your breath go from warmer to colder.

Why is this? I was told why by my next door neighbour of the time, a great bloke, and not one you’d contradict lightly. He’d told me stories of his time in Malaya, when he’d take suspects up in a helicopter three at a time, throw the first one out without asking anything, then ask the second for information and if he was reticent throwing him out, and finally not having to ask the third, who would always give the info voluntarily. My neighbour was obviously an expert in, say, psychology, but was he right with his physics? I did disagree with his explanation, party because since retiring from the services he no longer had access to a helicopter, and besides, like most people, he wasn’t that interested in what I might know.

His explanation was that with your mouth hole smaller, your breath was first pressurised, and then, as it exited your lips, it experienced a lowering of pressure. That loss of pressure, he said, would result in a decrease in temperature.

Although true to some extent, there are problems with this account, because the actual values of the temperatures, timings and pressures involved don’t work. First, the slight over-pressure caused by blowing through a small mouth hole will not last long enough for your breath to dissipate the increase in temperature it would experience. It would have to lose some of that heat before being blown out, in order to get much colder once blown out. If the other explanation were true, you would blow colder the longer you’d held the compressed air in your mouth. In fact, holding it inside would actually raise the temperature towards body temperature.

Instead, I suggest that “blowing cold” involves a narrow air jet, made with pursed lips of course, which entrains a sleeve of outside air along with it. The breeze that then hits your hand is mostly air that has never been inside you.

One of those Melvin was chatting to, agreed with my neighbour (about 18 mins into the programme), but I think they’re both wrong.

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One Response to Why does your breath get colder when your mouth hole is smaller?

  1. lesbionest mcguire says:


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