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How to be a dragon, or: why can I see my breath in the winter?

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Photo by Tomic Pasko via Flickr, used under Creative Commons

As a kid, my friends and I used to pretend we were dragons. On chilly days, we would blow clouds with our breath, pretending we were fire-breathers.  However, sometimes, even though it was cold out, no amount of breathing would form the “smoke” we wanted to see. Why does this cloud-breath only happen on some chilly days and not others?

When you exhale, you mix the warm and wet air of your breath with the air outside. On warm days, they mix together easily and nothing remarkable happens.  Likewise, when the weather is cold and relatively dry, the moisture from your breath will disperse into the air, and you won’t see a cloud.

However, if the air is cold and already pretty wet (like when the air is 95% saturated with water – which we call 95% humidity), hot, humid breath can add more moisture than the air can hold. If the combined humidity hits 100%, the air will “overflow” with the extra water. This excess water condenses into the cloud you see as your breath.

 And – as you might have noticed on those incredibly cold winter days when your teeth are chattering a lot more than usual – it’s generally easier to see your breath when it’s super cold than when it’s only somewhat cold. The colder the air is, the less overall moisture it can contain, so you need less water to reach the point of overflow.

Which means on any given day, knowing the temperature and humidity of the air outside, you can figure out if you will be able to see your breath: if it’s cold enough for water vapor to condense (around 45 degrees or lower), and you know you can add enough water vapor into the already humid air so its relative humidity reaches 100%, you can make as many clouds as you please.

So next time you want to play heroes versus dragons, or show off how many clouds you can make, be on the lookout for a cold and relatively wet day.

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