If you enjoy winter, then you probably like all the stuff that winter makes possibly, like skiing, sledding, making snowmen and building snow forts. But to do any of these things, you need snow! And winter does not always equal snow. What if there's not as much snow at your local ski hill as there usually is? Or what if you're hosting the Winter Olympics at a resort on the Black Sea? And what if you also don't have a sister who was born with the freakish magical power to freeze anything she wants? Then you have to make the snow, the non-magical way, the scientific way. And the science of snow-making is more complex than you might think.

(Intro)

Snow is precipitation composed of ice crystals. Unlike sleet or ice pellets, which is liquid rain that freezes on it's way down to the ground, snow forms when water vapour skips the liquid stage and condenses directly into ice, in clouds where temperatures are below zero degrees.

The ice crystal then grows into a snow crystal or snow flake as it absorbs and freezes more water vapour. So making snow is basically all about creating a heat exchange because heat has to be removed from water in order to transform it into ice crystals. For this to happen the ambient air temperature, the snow-makers call this the Dry Bulb Temperature, has to be really cold - even at zero degrees any manufactured snow is likely to be low-quality slush.

But equally important is the humidity, the amount of water vapour in the air. When the ambient temperature is adjusted to account for humidity it's known as the Wet Bulb Temperature. This is important because water cools by evaporating into water vapour. Your body uses the same process to cool itself, we call it sweating. 

So if it's too humid and the air is too saturated the water can't evaporate as well, which makes it harder to cool down. So snow makers hate high humidity almost as much as they hate low temperatures. They need both low air temperature and low humidity. When these conditions are at their sweet spot most snow machines, or snow guns, at your favorite ski area can combine cooled water and compressed air to create tiny water droplets and the smaller those droplets are the better because lots of little drops expose more of the water to the cold than few big ones. So snow guns use special nozzles to spray a fine mist that, snow gods willing, freezes almost immediately. To boost their chances most snow machines also shoot that mist high up in to the air or even use elevated snow lances or snow gun towers. These allow the super chilled water droplets to stay in the air longer so they have amble time to freeze and become that wonderful white stuff we need for skiing.

So it's not done the same way nature does it and it's not as easy as pointing finger at whatever you want to freeze. But the science is almost as fun, and frankly I don't care where it comes from or how it's made. I just want my snow because I want my Olaf! Do you wanna build a snowman?

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