Ask Sam: Why Can I See My Breath When A Car Drives By On A Cold Day?

Jan 11, 2019

Can you believe we found such a perfectly specific gif to illustrate this edition of Ask Sam?
Credit via Giphy /

Every other Friday on Morning Edition, NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call Ask Sam.

Michael from Ashfield, Massachusetts asks:

"I would like to know why on a cold day, if I’m outside and someone drives by in a car, I can see my breath all of the sudden, and then it goes away after a minute. I’ve been wondering this for a while. It also happens if I’m working around a chainsaw or something and I fire it up. All the sudden I can see my breath. "

I’m going to make a BOLD CLAIM here: Once you can answer this question, you will be equipped to explain a huge quantity of the natural phenomena that surround us from day to day. The knowledge that you will have once you can answer this question is SO POWERFUL, your eyes will never be closed again. There’s no turning back. 

Let’s start with some basic stuff. When you see car exhaust you are seeing water, and not water vapor. Like most gases, water vapor is invisible. The white fumes that come from a tailpipe is a cloud comprised of tiny droplets of liquid — or if it’s cold enough, solid — water suspended in the air.

That water is a byproduct of the combustion of gasoline, and it’s about 10 percent of the exhaust, and you see it because of condensation. Air from your exhaust is hot and hot air can hold more water, and when it hits the cold air outside of your car, it cools off and of the gaseous condenses into liquid water that you can see in a little cloud. In a well-maintained gasoline engine it should be the only thing you see.

Not coincidentally, this property of the universe — the fact that cold air can hold less water — is also why you can see your breath (which I will henceforth call mouth exhaust) on cold days. It’s the same dang thing: condensation.

So why-oh-why would a car driving past a few feet away lead to more condensation when you expel mouth exhaust out from your lungs? I puzzled through this with Jeff Naber, a professor of automotive engineering at Michigan Tech University.

The first explanation he posited was that you may be breathing in some of the un-condensed water coming from the car’s exhaust. When a car goes by, its exhaust is “generally raising the humidity locally around that car. So it may have not reached the condensation point, but as I breath it in, then i’m going to be closer to the condensation point,” he said. 

But this might be only part of the explanation.

There’s another possibility, which has to do with something called nucleation. For condensation to occur, molecules have to stick to each other and form up into a little ball, which happens most easily when there’s a little speck of something to start out with.

Credit Bruce Barrett via Flickr Creative Commons /

As a fun-science-experiment-slash-entire-genre-of-You-Tube-video you can super-cool very pure water to way below its freezing point and it won’t freeze until you drop something into the water to start the process off. The first ice crystal needs something to grab onto to in order to get started.

So explanation number two is that the particulates — the tiny chunks of pollution in the car exhaust — are providing that little nucleus for the water in your breath, and thus the temporary increase in cloudy mouth exhaust.

But here’s a really tricky twist: It might be that you’d see your breath more clearly even if an electric car drove by, even though they have no emissions from their tailpipes. Why? Because cars kick up all sorts of tiny shmutz.

“It’s just as possible that the car tires kicked up more dust and the dust increased the nucleation of the water,” Jeff explains.

You might not even be breathing all of these dust particles in, if the air in front of your mouth is dusty, those are little nuclei that will help to form clouds from your mouth exhaust!

I love thinking about condensation and nucleation. The behavior of water out in the world explains so many little things that we see every day, like, why is your grass covered in dew some mornings and not others? (Relative humidity!)

Why do you have to scrape a ton of frost off your windshield and not others? (Same!)

Why do you get fog in river valleys some days? (Latent heat of vaporization!)

Why do you get more clouds over the ocean? (Nucleation!)

Why are the insides of your windows wet or frosty on really cold days? (Condensation!)

When you walk through the world alert to these tiny changes it’s just a much more interesting way to pass through life than just being grumpy whenever you have to scrape ice off your windshield.

(I told you there would be no turning back!)

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