If I pour boiling water into a mug and throw it out from the balcony in very cold weather (-20 degrees Celsius and below), then it instantly turns into a cloud of steam, even before it reaches the ground. But if you do the same thing in warm weather, the water will fall to the ground as a liquid.

Why is this happening? How does cold cause instantaneous evaporation of boiling water?

  • $\begingroup$ Clouds are not steam. $\endgroup$
    – DJohnM
    Nov 1, 2023 at 17:24

2 Answers 2


As a kid this question really bothered me!

The phenomenon you're describing, where boiling water turns into a cloud of steam when thrown in very cold weather, is a result of the Leidenfrost effect combined with the extreme temperature difference between the boiling water and the cold environment.

Here's what happens:

Boiling Water: When you pour boiling water into a mug, the water is at or near its boiling point, which is typically around 100 degrees Celsius (212 degrees Fahrenheit). The water molecules are moving very rapidly, and some of them are in the gaseous state (water vapor).

Leidenfrost Effect: When you throw this boiling water into very cold air (e.g., -20 degrees Celsius or below), a layer of water vapor quickly forms around the droplets of water. This is due to the Leidenfrost effect, which occurs when a liquid comes into contact with a surface that is significantly hotter than its boiling point. In this case, the extremely cold air acts as that surface.

Vaporization: The layer of water vapor insulates the liquid water droplets from the cold air, preventing them from freezing instantly. Instead, the water droplets start to vaporize at a rapid rate due to the large temperature difference between the hot water and the cold air. This rapid vaporization creates the visible cloud of steam that you observe.

The Leidenfrost effect is why the water doesn't immediately freeze on contact with the cold air but forms a cloud of steam instead. The effect is more pronounced when the water is hotter, and the temperature difference with the surrounding environment is greater.

In warmer weather, the temperature difference between the boiling water and the air is not as extreme, so the Leidenfrost effect doesn't play as significant a role. As a result, the water is more likely to cool down more slowly and reach the ground as a liquid.

  • 3
    $\begingroup$ AI generated, right? $\endgroup$
    – Ghoster
    Nov 1, 2023 at 4:35
  • 1
    $\begingroup$ "the water droplets start to vaporize at a rapid rate due to the large temperature difference between the hot water and the cold air" but how does it happen? How temperature difference vaporizes droplets? $\endgroup$
    – Robotex
    Nov 1, 2023 at 16:13

The water is freezing and forming ice!

The3re are many videos on the Internet showing this phenomenon and here is one.

  • $\begingroup$ In other words, the water that evaporates becomes ice and is visible. In warm weather, there is more evaporation but you can't see it. $\endgroup$
    – Daniel
    Oct 31, 2023 at 13:27
  • $\begingroup$ The ambient temperature is so low that any water that evaporates, ie becomes water vapour, almost immediately freezes and the rest of the water liquid just freezes. In warmer weather the rate of loss of heat from the water is much less and the water stays as colourless water vapour. The "opposite" effect is when you boil a kettle of water the steam (water vapour) condenses out to small water liquid droplets which you observe as being white due the light scattered from them.. $\endgroup$
    – Farcher
    Oct 31, 2023 at 13:43

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