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Breathing Hot and Cold

One of the most common questions that I get is where do I get the ideas for these experiments. Some are old classics that I try to give a new angle. Others are the result of questions sent to me by subscribers. Some of the ones that I like the best are the ones that just pop up, seemingly out of nowhere. This is one of those. It also comes with its own story, which makes it even better.

The story is one of Aesop's fables. If you have never read any of these, go read some. It is well worth the time.

The story we are interested in is The Man and the Satyr. What is a Satyr? It is an imaginary creature like a man, with the legs and horns of a goat and the ears and tail of a horse. Quite a strange thing to imagine.

The Man and the Satyr

A Man and a Satyr once drank together in token of a bond of
alliance being formed between them. One very cold wintry day, as
they talked, the Man put his fingers to his mouth and blew on
them. When the Satyr asked the reason for this, he told him that
he did it to warm his hands because they were so cold. Later on
in the day they sat down to eat, and the food prepared was quite
scalding. The Man raised one of the dishes a little towards his
mouth and blew in it. When the Satyr again inquired the reason,
he said that he did it to cool the meat, which was too hot. "I
can no longer consider you as a friend," said the Satyr, "a
fellow who with the same breath blows hot and cold."

Recently, I heard a news story that referred to a politician as blowing hot and cold with the same breath and it made me think of this fable. That got me thinking about the science behind it. How can you blow hot and cold?

To try this, you will need:

  • several drinking straws
  • a sheet of notebook paper
  • a mirror

First, hold your hand about 3 inches in front of your mouth. Purse your lips, as if you were going to blow out a candle and instead, blow on your hand. What do you feel? Quite cool, right?

Keep your hand at the same distance. Open your mouth wide and breath on your hand again. Does it feel cool this time? No, instead it feels warm. How can that be?

My first thought was that it had something to do with air speed, so I tried both ways at different speeds. Holding your hand at the same distance, purse your lips and blow gently. Then blow faster. Try the same thing with your mouth open. No good. Slow or fast, with your lips pursed, it feels cool and with an open mouth it feels warm.

After quite a bit of thought, the only thing I could see that was different was the size of the opening the air was coming through. Let's check to see. We will simulate pursing your lips by blowing through a straw, since it also has a small opening. Hold your hand about 3 inches from the end of the straw and blow through it. OK, it still feels cool.

Next, roll the sheet of paper into a tube about the same diameter as your mouth was when it was open wide. Again holding your hand about 3 inches from the end, blow through it. Ah, it feels warm. We are making progress.

At this point, I was thinking that with a thin stream of air, it was cooling quickly as it moved through the surrounding air, and that a thicker stream of air was cooling at it outer edge but staying warm in the center. If that was true, then blowing through pursed lips or a straw at very close range should feel warm, as the air would not have had time to cool. Try it. If you hold your hand about half an inch from your lips, it feels warm even with pursed lips. Looking at things in reverse, the air from an open mouth should feel cool if it is far enough away. You may have to blow very hard, but you will see that this is also true.

Taking that one step farther, I used a bundle of drinking straws. As a bundle, it carries the same amount of air as the tube of paper, and it works the same way. Holding your hand about 3 inches away, the air feels warm when you blow through it, but if you hold the back of a finger at the same distance, you can find more detail. If it is hit by the air from one of the center straws, it feels warm, but the straws on the outside give a breeze that feels cool.

That just leaves the cause of the warm and cool sensations. Try repeating each of the experiments using a mirror instead of your hand. For the tries that made your hand feel warm, your breath fogs the mirror. For the ones where you felt cool, there is no fog, and if some was already there, it goes away quickly. As moisture condenses, it gives up heat, making the air around it warmer. The air from your lungs is already warm. Condensation causes it to warm up a bit more. At that point, it feels warm on your hand.

As your breath moves along, it mixes with the surrounding air. It begins to cool and gives up some of its humidity. At that point it is just like the air from a fan. It moves across your hand and evaporates some of the moisture on the skin. That causes it to feel cool.

That sounded logical, so I wrote it up and sent it to Dr. Kim Aaron, an aerodynamicist friend at NASA's Jet Propulsion Lab. He helps with experiments now and then, and he always comes up with interesting angles to things. He confirmed that I was on the right track, making several suggestions that made it easier to write this. So easy that this is getting far too long, so I will stop here. On the other hand, you are free to continue exploring as long as you want. Just don't huff and puff so much that you start seeing satyrs.