Every winter, one of the coolest trends to circulate around the web is courtesy of unfortunate, bored souls stuck in the northern tundra with nothing better to do than throw cups of boiling water into the air. Once the water leaves the cup, it immediately turns into "snow." Let's take a look at how and why that happens.
First off, what's really happening isn't the formation of snow, but the water is rapidly evaporating into vapor that immediately condenses into a cloud. I tried the boiling water trick last night at The Vane's nerdquarters in North Carolina, where the air temperature at the time was 11°F and the dew point a dry -18°F. As soon as I threw the water into the air, the liquid disappeared in a big, short-lived cloud.
Why does this happen? Boiling water is closer to evaporating than cold water, so when you throw extremely hot water into a very cold, very dry atmosphere like the one hugging the eastern half of the United States right now, the smallest droplets are able to cool and evaporate in a dramatic cloud before they reach the ground. The experiment works best when the air temperature and dew point are far below zero, but even a modestly frigid 11°F/-18°F combination like we had in North Carolina at 1:00 AM does the trick.
Much like condensation trails, the relative humidity of the atmosphere is key in determining how big the cloud gets and how long it sticks around. An 11°F air temperature and -18°F dew point creates a relative humidity of about 25%—in other words, very dry air that doesn't let the cloud hang around for more than a few seconds, at most.
The most viral example of the boiling water experiment made the rounds last year courtesy of a resident of Novosibirsk, Russia, a city in southwestern Siberia. The guy in the video throws a pot of boiling water off of his seventh-floor balcony in -41° (both C and F) temperatures, and the result is spectacular.
The weather is awesome, and it's even more fun when it's interactive. However, if you try this at home, be careful of which way the wind is blowing and where you throw the water. What water doesn't evaporate won't instantly cool down, so any liquid that reaches the ground (or your arms, or your face) will still be hot enough to cause some serious burns.