Anyone who has ever skied early season conditions knows the love-hate relationship that skiers have with manmade snow. While it isn’t exactly as nice to ski on as real snow, it allows us to ski months earlier than we otherwise would! So how does snowmaking work?
When temperatures are cold enough, snow guns spray small water droplets into the air, which freeze and fall as “snow”. In reality, this “snow” is actually a bunch of tiny ice pellets, since this method does not create the same snowflake shape that true snow has. This is why manmade snow doesn’t ski as well as real snow, since it packs down into a tighter space, which changes the texture.
So how cold does it need to be to make snow? 32F or 0C, right? Actually, no. The answer is a bit more complicated and revolves around a concept called evaporative cooling.
Essentially, evaporative cooling describes the cooling of an object due to water vapor. For example, say the air temperature is 50ºF, and you and your friends are going swimming. Before you get in, the air feels nippy but not terrible. However, once you get out before you dry off, the air feels much, much colder. This is due to evaporative cooling, which revolves around the fact that it takes thermal energy to turn water into water vapor.
The same principle is true for falling snow. Moisture in the snow crystal actually lowers the temperature as it falls, allowing snowflakes to fall even when temperatures are above freezing.
We actually have a way to measure evaporative cooling: the wet-bulb temperature. This term originates from the way it was originally measured by wrapping a wet towel of sorts around the bulb of a thermometer, which allowed the thermometer to measure the temperature of the air with evaporative cooling factored in. Some weather services may give the wet-bulb temperature, but it’s straightforward to calculate on your own if they don’t. You can use a calculator like this one to obtain the wet bulb temperature from temperature and relative humidity.
As long as the wet-bulb temperature is below about 28F, snowmaking is possible, allowing us to ski at our favorite resorts in October and November!