The science behind tossing boiling water into minus-14-degree air

Most articles about physics don’t need to start with a disclaimer, but this one does.

Please — for the sake of your well-being, reputation and the U.S. healthcare system — do not throw a pot of boiling water into the wind if the wind is blowing at you. If you read this story and suddenly feel inspired, please take two seconds to think about your surroundings and about whether you really want your buddy Instagramming what you’re about to do.

As a polar vortex brought subzero temperatures to much of the Central U.S. on Wednesday, Midwesterners have once again embraced a cherished internet-era tradition of throwing boiling water in the freezing air and posting videos of the results — a spectacular white cloud — on social media. It’s the Arctic version of trying to fry eggs on car hoods during heat waves.

Provided you don’t hurt yourself or others near you (like dozens of people did last time it got this cold), the boiling water trick is nice a do-it-yourself physics experiment that can be a useful lesson about the properties of heat and water. The Times talked to a couple of physicists about what’s really happening.

Is the water freezing midair?

“A lot of people say boiling water freezes immediately, but that’s not what’s happening,” said Jeff Terry, a professor of physics at the Illinois Institute of Technology in Chicago. “It’s not an instantaneous freezing of the water.”

In a video he tweeted Wednesday, Terry threw a pot of boiling water into minus-14 degree air, creating a big white cloud that drifted away, with dozens of small contrails streaking toward the ground.

The big cloud is condensate, or water that has quickly condensed into tiny droplets, which is the same reason you can see your breath when it’s cold outside. The smaller streaks are from condensate coming off falling water droplets — not water that has frozen midair.

The air is not quite cold enough to freeze water immediately, which happens at about minus-42 degrees, Terry said.

“It’s not snowing down,” he said. “You don’t see ice crystals falling to the ground. … Theoretically, if it were colder, you could actually see that.”

Does it really matter if the water is hot? Minus-14 degrees is pretty cold.

Yes.

“Boiling water or hot water evaporates much, much more rapidly than cold water,” said Jonathan I. Katz, a professor of physics at Washington University in St. Louis. The amount of vapor “increases very rapidly as the temperature goes up.”

For that reason, throwing boiling water into the air will look more impressive than throwing room-temperature water. More of it will evaporate, making a bigger cloud.

Nor do you get the same effect when it’s warm outside. Cold air can’t hold as much moisture as warm air.

I’ve heard that hot water freezes faster than room-temperature water. Is that what’s happening?

No, the important principle in the boiling-water trick is how fast the water evaporates, Katz said.

Any advice?

“Try and disperse the water as much as you can,” to maximize evaporation, Terry said.

Oh, and another thing: “You never want to throw it into the wind.”

“You gotta think this through just a little bit before you do it,” Terry said. “It is kind of a cool science experiment, right? If you do it carefully, you can do this without really a tremendous amount of risk to yourself. But that’s always the problem, that people don’t do things carefully.”

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Matt Pearce is a national reporter for The Times. Follow him on Twitter at @mattdpearce.

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