You've been doing it since you were a little kid. On a cold winter day, you take a deep breath, let it go, and watch big foggy billows come out of your mouth and float away. It's like you're making clouds! But why doing it happen? Why can you see your breath when it's cold out?
Well, it's because the moisture in your breath is experiencing a rapid change in dew point. Dew point is the temperature at which water vapour starts to condense out of the air and forms a liquid just like the dew on your front lawn. When air's warm, like in the summer or inside your body, the dew points are high because heat makes water molecules move around fast and turn in to gas. So warmer air can hold more water, that's why you get so muggy in the summer. But cold air slows these molecules way down and they begin to condense. As a result, lower temperatures often mean less moisture in the air.
Now our lungs, no matter what time of the year, are kinda like Georgia in July – warm and muggy. When you exhale, the air leaving your body is actually completely saturated with moisture. That means the relative humidity of your breath is 100%. And when the water vapour in this warm, sultry, saturated air strikes the cold air, those water molecules slow down and quickly condense in to liquid. It's the same mechanism that causes fog. Masses of warm, humid air striking cold air causing condensation.
But here's the thing: in order to condense, those molecules need something to stick to. So they actually attach themselves to tiny particles in your breath called 'condensation nuclei', or even 'cloud seeds'. These provide a solid platform for water vapour to condense into a droplet and your cloud seeds are composed of all kinds of things. Soot, dust, cell particles from your lungs, even bits of your proteins and DNA. In fact, scientists are developing a way to analyze our exhaled breath condensate, or EBC as it's called, to look for the presence of air pollutants or signs of respiratory illness. But when you see your breath on a cold day, it's born from the same phenomena that form clouds and fog and dew.

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