One winter I was in the airport in Minneapolis, Minnesota getting ready to fly home. I had arrived there on a flight from Rochester, Minnesota in the afternoon. As the sun set, however, there was a problem – it got so cold they closed the airport. It was something amazing like -40 degrees F. So the airline put us up in a hotel across the street from the Mall of America, and it was from the valet at that hotel that I first saw the trick of turning boiling water into instant snow. He was performing the trick for tips.
It’s a simple trick. You take a cup of boiling water and throw it into the air. It turns to snow instantly, as shown here:
In the infrared shots you can see how rapidly the water cools. It disappears from the infrared image because it has become the same temperature (or close) as the air.
I’m sure Minnesotans see this trick all the time, starting when they are toddlers. But if you are from the southeastern U.S., you’ve never seen it and it is amazing.
The obvious question is: why boiling water? Wouldn’t this work even better with cold water? Apparently not. Here is one page that tries to explain the reason:
Be sure to use hot water. Hot water evaporates more quickly than cold water; that quicker cooling effect makes the ice shell form more quickly at the surface of each drop…. the hotter the water, the faster the water molecules at the surface of a droplet evaporate. That creates an enhanced cooling effect on the surface of the droplet, which makes it freeze more quickly.
Although I have not run across this explanation, I would guess that another part of it has to do with the viscosity of boiling water. Everyone knows that oil, honey, molasses, etc. flow better when warm. That’s because the warmth lowers the viscosity (see this page for an explanation of how viscosity works). Water works the same way, as shown in the first table on this page. The hotter the water, the lower the viscosity. I imagine that hot water breaks into finer droplets, which speeds freezing (because the droplets are so small).
Let’s also keep in mind that, if it’s cold enough, it probably doesn’t matter what the temperature of the water is when you throw it. In “To Build a Fire”, Jack London talks about spit cracking in the air before it hit the ground because the air is so cold:
As he turned to go on, he spat speculatively. There was a sharp, explosive crackle that startled him. He spat again. And again, in the air, before it could fall to the snow, the spittle crackled. He knew that at fifty below spittle crackled on the snow, but this spittle had crackled in the air. Undoubtedly it was colder than fifty below—how much colder he did not know.
In those conditions its probably best to stay inside.
See also: How Snow Makers Work