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Chapter 18: Problem 10

Blow on the back of your hand with your mouth wide open. Your breath will feel hot. Now tighten your lips into a small opening and blow again. Now your breath feels cool. Why?

Short Answer

Expert verified
The breath feels cool when blowing with a small mouth opening because the air being expelled mixes more readily with surrounding air, cooling it more quickly. When the mouth is wide open, there is less mixing and thus, the breath retains more of your body heat, causing it to feel warmer. The principle of cooling is at work here, where the rate of heat transfer is proportional to the temperature difference between the two interacting entities.

Step by step solution

01

Recognize the difference in breath temperature

Before we start, notice the difference in the temperature sensation when you blow air with an open mouth versus with your lips puckered. The air is certainly the same temperature, as it comes from your lungs, but it feels different. This provides a crucial piece of information which is used to understand the underlying cause.
02

Understand the effect of air speed

When you blow air with your mouth wide open, the expelled air mixes to a lesser extent with the surroundings, hence it retains more of your body heat. Thus, the breath feels warm. But when you blow air with your lips tightened into a small opening, the air is expelled at a higher speed, it readily mixes with ambient air and thus, gets cooled quickly. This is why it feels cool.
03

Discuss the principle of cooling

The speed at which heat is transferred from a hotter object to a cooler environment is proportional to the difference in temperature between the two. So, cooler air, coming into contact with the warm air you are exhaling, causes more rapid cooling when expelled with pursed lips.

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