science knows why

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There goes the question, so verbatim, as I found it the other day: “Why is it that when we blow “fuuu” cold air comes out and when we blow “haaa” hot air comes out?”. That is to say, why does the air that we expel through the mouth changes temperature depending on the size of the mouth opening? And I share it not only because it has kept me going “haaa” and “fuuu” for an entire afternoon, but because it is a good example of how intricate physics, physiology and everyday life are.


Are we able to heat and/or cool as we please?. Although it may not seem like it, the question has a crumb. If it’s cold, we use our breath to warm our hands. The same breath we use to cool down a cup of coffee or a spoonful of overly hot soup. That is, we might think that we have the capacity to produce hot or cold air as it suits us. The problem, of course, is that this is not exactly the case.

It is not exactly like that because before leaving our mouth the air always has the same temperature: that of our body. Although several factors intervene, what changes is the way we handle air flows and how the most elementary physics interacts with them.

Resolving the great debate of each summer: the dry heat of the interior or the humid heat of the coast is worse

Don Daniel Bernoulli Parera. When we blow air, many physical phenomena take place, in fact. An especially relevant one is, for example, the Bernoulli effect. The same principle that keeps planes flying in the sky. What Daniel Bernoulli deduced in 1738 is that as the speed of an air flow increased, the pressure in it was reduced. The jet of air becomes a natural “attractor” of all the air that is in the room and, since we are normally in rooms that are colder than our body, the jet tends to cool down.

Precisely for this reason, we do not notice the cold of the jet if we stick our hand to our mouth while we blow. And, precisely for this reason, when we make “haaa” with our mouths open (by not generating a small jet of air, but something much larger and indefinite) the dough takes longer to cool down.

The places where this doesn’t work. There are more things involved, of course: like the relative amount of moisture in our lungs is much higher than that outside. When we blow on our skin, this humidity generates the cooling phenomenon that we talk about so much in summer. Sweat, let’s remember, is our main biological cooling system. All this is what causes that, in a sauna (with high temperatures and 100% humidity), the “fuuu/haaa” phenomenon not occur: blowing on your skin gives heat.

little big questions. Daily life, although we do not see it, is full of principles, phenomena and physical, chemical, physiological and psychological effects that order the day to day. What happens is that, often, we do not see them. That is one of the wonders of contemporary science… that it never tires of illuminating our lives with new perspectives.

Image | Julien L.

There goes the question, so verbatim, as I found it the other day: “Why is it that when we blow…

There goes the question, so verbatim, as I found it the other day: “Why is it that when we blow…

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