Boiling Water Into Snow: The Science of Hot Water in Extreme Cold

Toss a pot of boiling water into the air on a bitterly cold day and something magical happens: instead of splashing on the ground, it bursts into a roaring white cloud that drifts away like a puff of snow. In the clip Make it snow!, uploaded by 125fors in January 2016, the uploader reports an air temperature of −36 °C.

How boiling water turns to snow

Climatologist Mark Seeley of the University of Minnesota explained the trick to Live Science. It depends on a huge temperature difference between the water and the air, and on a few simple steps of physics:

1. Boiling water evaporates fast. That is why it steams. Water near 100 °C gives off a great deal of vapour very quickly.
2. Throwing breaks it into droplets. Tossing the water splits it into countless tiny drops, which hugely increases the surface area for evaporation.
3. Very cold air has almost no room for vapour. Cold, dense air can hold only a tiny amount of water vapour, so the sudden flood of moisture pushes it past saturation.
4. The vapour freezes onto tiny particles. The excess vapour condenses and freezes onto microscopic particles in the air, such as sodium or calcium compounds, forming ice crystals. Seeley points out that this is the same process that forms natural snowflakes.

How cold does it have to be?

The famous video Seeley discussed was filmed in Canada’s Northwest Territories at about −30 °C (−22 °F). Seeley himself waits for around −34 °C (−30 °F) before trying it in Minnesota. He says −30 °C can work if the air is very dry, with extremely low relative humidity.

The water has to be hot. Cold water gives off far less vapour, so it cannot flood the air with moisture in the same way, and most of it simply falls back down. Boiling water, with its rush of vapour, is what makes the instant cloud.

Is this the Mpemba effect?

No. The Mpemba effect is the claim that hot water can freeze faster than cold water in a container, under controlled conditions. The boiling-water cloud is a different thing: most of what you see is vapour turning straight to ice crystals in the air. Read more in our pieces on The Mpemba Effect: Why the Experiment’s Definition Matters and Instant Ice: What Supercooled Water Demonstrations Reveal.

Watch it, and keep the boiling water away from people

Larger drops in the plume can still be scalding when they come down. During the 2019 cold spell in North America, Sky News reported people being taken to hospital after the boiling-water challenge went wrong. Enjoy the videos, and leave the real thing to people who know exactly where the water will land.

For safer winter experiments, try the frozen soap-bubble film or Minnesota Cold’s frozen flowers.

Related reading

• Coloured Ice Orbs: A Winter Craft with Water and Balloons
• Frozen Bubbles at Mount Washington Observatory
• Soap Bubbles in a Thompson, Manitoba Winter
• Hot Water in Extreme Cold: What the Pale Cloud Shows
• Dry Ice in Water: Understanding the Low White Cloud
• Chris Ratzlaff’s Frozen Soap Bubbles in Calgary

Sources