Question: How come you can see your breath when it's very cold outside and not when it's hot?

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  1. Your breath contains water – gaseous water. Air is wet anyway, but when it comes out of you, it contains more water. You’re quite a watery thing.

    You breathe the water out as a gas. When it hots cold air, the gaseous water condenses to a liquid because it cools down and becomes water. The gas becomes little droplets of water – like clouds. So you can see it because the water droplets scatter light better than the gas. Hot air doesn’t make the gaseous water condense.

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  2. Condensation. When we breathe out there are various gases being expelled, including water in a gaseous state. We can see this water when we breathe out on a glass surface or mirror. The water condenses (cools) to a liquid form as it has just left your very warm insides for a relatively cool surface.

    On a cold day the same things can be seen when you breathe out into the air. Warm gaseous water from your lungs hits the cold air and condenses into tiny droplets of water which hang in front of your face.

    The same effect isn’t seen on warm days because the temperature difference between the inside of your body and the outside air is not enough to cause condensation.

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