People on the Internet do like to ask a ridiculous question. One such question, posed several times throughout recent Internet history, has become a meme classic; “Why does ha make hot but hoo make cold? I’m talking about mouth wind”.
In less Internet-speak terms, the question is really asking why you are able to produce both warm and cool air from the largest of your face holes, the mouth.
Hot air is pretty simple to explain. Your body, assuming you aren’t a cold-blooded reptile that has somehow learned to read, is warm. As a result, the air you expel from your body is slightly warmer than the surrounding environment. Hold your hand close to it and breathe on it, and it will feel warm.
But if you purse your lips and let rip a “hoo” from a distance, it will feel cool. This is the result of a process known as entrainment. Essentially, pursing your lips and forcing out air creates a narrow jet. The jet itself is still warm close to your mouth. Try pursing your lips and blowing right next to your hand, and see what that feels like.
Fast-moving jets in fluids (including gases) produce lower pressure areas around them, according to Bernoulli’s principle. This low-pressure area draws in surrounding air, as can be seen in this bag-blowing demonstration.
With entrainment, the jet of air from your “hoo” drags cooler air along with it, and it is this that cools down your hand (assuming it is far enough away from your mouth).
But why does fast-moving air heading at your hand make it feel cooler rather than warmer?
“The reason is because as wind blows across our bodies it takes the heat we naturally emit and blows it away from our bodies,” the National Weather Service explains. “The faster the wind speed the faster our body heat is taken away and the colder it feels. It is a similar process for when you blow on a hot bowl of soup to cool it down.”
So that’s why haa is hot, and hoo is cold, when talking about mouth wind.
Source Link: People Are Asking Why "Haa Makes Hot But Hoo Makes Cold"
