Throw a pot of hot water into the air at minus 30 °C and it explodes into a white cloud that drifts away like smoke. This short clip, filmed in Yellowknife in Canada's Northwest Territories, shows one of the most popular cold-weather experiments in the world, and the physics behind it is simple once you see what the cold air is doing.
Why boiling water turns into a cloud of ice
Two things have to meet: very hot water and very cold, dry air. Boiling water gives off vapour readily — that is why it steams. Throwing it breaks it into tiny droplets, which multiplies the surface area for vapour to escape. Mark Seeley, a climatologist at the University of Minnesota, told Live Science that this creates "even more surface for water vapor to come off of."
Cold air, meanwhile, can hold very little water vapour. When the burst of vapour hits it, the air is instantly oversaturated. The excess vapour condenses and freezes onto microscopic particles in the air, forming ice crystals — the same basic process that makes snowflakes in a cloud. The white plume you see is that mass of tiny droplets and crystals scattering light.
Why cold water just falls as water
Pour cold tap water into the same air and it mostly splashes on the ground. Cold water releases far less vapour, so there is little to condense and freeze, and the larger drops stay liquid long enough to land. It sounds backwards, but hot water is the key ingredient.
Seeley noted that he usually waits for about minus 30 °F (minus 34 °C) in Minnesota before trying it, and was surprised it worked at minus 30 °C in Yellowknife. Very low humidity helps, which is why the trick succeeds at slightly milder temperatures in dry northern air.
Yellowknife: a city built for the cold
Yellowknife sits on the northern shore of Great Slave Lake, about 400 kilometres south of the Arctic Circle. It was founded in 1934 after gold was found nearby and became capital of the Northwest Territories in 1967; around 20,000 people live there. The city takes its name from the Yellowknives Dene, and in the Dogrib (Tłı̨chǫ) language it is Sǫǫ̀mbak'è, "where the money is".
January averages around minus 26 °C, and the record low is minus 51.2 °C, set on 31 January 1947. Winter also brings ice roads, including the 6.5-kilometre road across the lake to Dettah, and long, clear nights that draw visitors from all over the world to watch the northern lights.
Minus 30: Celsius or Fahrenheit?
Cold-weather videos often mix the two scales. Minus 30 °C equals minus 22 °F, while minus 30 °F is about minus 34 °C. Only at minus 40 do the two scales meet. Keeping the unit next to the number makes reports from Yellowknife, Fairbanks or Minnesota easy to compare.
Stand with the wind at your back if you try it: the water is still scalding when it leaves the pot, and any that falls back as liquid can burn.
Explore more: the white fog around dry ice; freezing bubbles without a hot-water throw.
Related reading
- Spectacular Photos of Bubbles Frozen in Frigid Temperatures
- Québec City at Christmas: Old Streets and the Wider Winter Season
- The Ice Left by a Chicago Warehouse Fire Response
- Abraham Lake’s Ice Bubbles: Layers beneath the Surface
- Silverthorne’s Ice Castle: Tunnels, Arches and Frozen Light
Sublimation is the conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. For those of us interested in the water cycle, sublimation is most often used to describe the process of snow and ice changing into water vapor in the air without first melting into water. The opposite of sublimation is "deposition", where water vapor changes directly into ice—such a snowflakes and frost.
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