Toss a pot of just-boiled water into air at −41°C and it never reaches the ground as liquid. It explodes into a hissing white cloud that drifts away like smoke, while fine ice crystals fall out of it. A short RT clip from a Russian winter showed the trick at its most dramatic, and the physics behind it is simple, elegant and well understood.
Why boiling water turns to snow in extreme cold
Three things happen almost at once. First, water near 100°C is already releasing vapour fast, because its molecules are moving quickly. Second, flinging it into the air shatters it into countless tiny droplets, multiplying the surface area from which vapour can escape. Third, very cold air can hold only a tiny amount of water vapour.
The result is a sudden excess of vapour that the frigid air simply cannot carry. As University of Minnesota climatologist Mark Seeley explained to Live Science, that vapour condenses and freezes onto microscopic particles in the air, forming ice crystals by the same process that builds snowflakes in a cloud. The white plume is a mix of condensed fog and freshly made ice, and the smallest droplets freeze in the air before they can land.
How cold it needs to be
The colder and drier the air, the better it works. Minnesotans traditionally wait for about −30°F (−34°C) before trying it, though Seeley noted it can succeed at −22°F (−30°C) when the air is very dry. Iowa PBS filmed a version at −15°F (−26°C) with a −20°F dew point and still got a dense cloud. At −41°C, as in the Russian clip, the effect is about as strong as it gets: almost nothing reaches the snow below as liquid. For reference, −40 is the point where the Celsius and Fahrenheit scales meet.
Cold tap water will not do the same thing. It releases far less vapour, so it falls mostly as droplets or splashes onto the snow. The spectacle depends on the huge temperature gap between the boiling water and the air.
The Mpemba effect and hot-water myths
Demonstrations like this are often linked to the Mpemba effect, the claim that hot water can freeze faster than cold water. It is named after Erasto Mpemba, a Tanzanian schoolboy at Magamba Secondary School who noticed in 1963 that his hot ice-cream mixture froze before his classmates' cooled ones. In 1969 he and physicist Denis Osborne published the observation in the journal Physics Education.
The Mpemba effect concerns containers of water cooling in a freezer, and physicists still argue about when and why it appears. The boiling-water cloud is a different and much clearer phenomenon: it is about evaporation and condensation in the open air, with tiny droplets doing most of the work.
Burn risk and safe viewing
Enjoy this one on screen. During the 2019 polar vortex, the "boiling water challenge" went viral in the United States, and Loyola University Medical Center near Chicago admitted eight people, aged 3 to 53, to its burn centre after attempts went wrong. Burn surgeon Dr Arthur Sanford warned: “There is no safe way to do it.” If you want to see the effect, watch it on video.
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