Weather Blog

How can dry air make it snow more?

How can dry air make it snow more?
It's a strange contradiction -- having dry air in place can increase your chances of snow when temperatures are on the cusp of freezing. How is that possible? Through a process called "evaporative cooling."

Evaporative cooling is when moisture first falls from the clouds and into a layer of very dry air. Since the air has plenty of room to hold the moisture, the rain or snow will evaporate into water vapor. However, evaporating water requires heat and energy. As more and more drops evaporate, the air is using up more of its heat and energy, thus making the air cooler. (In addition, the process of melting snow as it initially falls into the warmer air mass also requires energy and in essence, removes heat as well.)

You can get a sense if this is possible by looking at the current temperature and dew point. (Dew points are the temperature at which the air would be saturated. If you take the mid point of the dew point and temperature, and it’s 32 degrees or less, then the precipitation could begin as snow. (For example, if it’s 40 degrees with a 20 dew point, that’s a midpoint of 30 degrees. That's because the temperature drops as the air cools, but as the air gains more moisture, the dew point rises. They generally meet somewhere close to the middle.)

Tuesday's snow was greatly aided by this process. To wit: at Gig Harbor, it was 34 degrees at 5 a.m. with a 17 degree dew point. Once it began snowing the temperature dropped to 31 degrees but the dew point rose to 25.

At Sea-Tac, it was 34 degrees with a 21 dew point just before the snow began at 5 a.m.  34 degrees is typically just a fringe snow situation, but two hours later as that evaporative cooling and melting mechanism took hold, the temperature dropped to 30 degrees with a dew point of 28.