Why you can make snow even if it's not freezing outside

Cory Reppenhagen explains why.

Warm days Tuesday and Wednesday has set the snow-making back for Colorado's ski areas, but colder nights starting this weekend will put those operations back into full swing.

Did you know that the air temperature doesn't even have to be freezing to make snow?

The key to frozen precipitation is whats called the wet bulb temperature. That is the temperature the air would be if you completely saturate it with moisture.

The snow-making cannons they use at places like Arapahoe Basin Ski Area can do their thing even if the air temperature is 36 or 37 degrees, as long as the air is really dry. 

For example: If the air temperature is 37 degrees, but really dry, say with a dew point of 18 degrees, then the wet bulb temperature is actually below freezing at 31 degrees, and snow can now be created. That’s because when moisture is being pumped into dry air, evaporation takes place, and evaporation is a cooling process.

Alan Henceroth, chief of operations at Arapahoe Basin, says they target a wet bulb temp of 28 degrees for optimal snowmaking. He says they are looking forward to a good weekend of snowmaking with cold nights in the forecast.

This principal is the same in nature, and is one of the reasons it can still be snowing when the air temperature is above freezing. With very dry air in place, it can start raining at say 36 degrees, and as that rain evaporates, the air cools, and it turns to snow, without any other cold air being moved in. The other common way to get snow when temperatures are above freezing, is to have a very shallow warm layer at the surface. Snowflakes falling from the cloud don't have enough time to melt before they hit the ground.

You can figure out what the wet bulb temperature is without using a wet bulb thermometer. It is a very simple calculation called the 1/3 rule. Take the air temperature and subtract the dewpoint. That is whats called the dewpoint depression. Now divide the dewpoint depression by 3, and subtract the result from the air temperature. You now have an approximation of the wet bulb temperature.

Next time the temp is not quite freezing, but the air is really dry, just think of the snow that could still be created. Artificially or naturally.

As for Arapahoe Basin, they are ahead of last years pace when it comes to snow making.

It would be tempting to get the Basin open this Sunday, that would be the earliest they’ve ever opened, but Alan Hencecoth says that looks very unlikely at this point. They will be relying on Mother Nature to come through with cold temps, but not necessarily freezing temps.

© 2017 KUSA-TV


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