I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook. There's more than one way for Mother Nature to get rid of a fresh blanket of snow. The most common way, of course, is by melting-which gives everyone the pleasure of trudging through slush, mud, and water. But in the western U.S., there's a wind called the Chinook, or "snow eater," that vaporizes snow before it even has a chance to melt.
Chinook winds are westerlies from the Pacific whose moisture gets wrung out as it passes over the Rocky Mountains. Once these winds come down from the mountains onto the high plains, they can be quite mild and extremely dry-as warm as 60 or 70 degrees Fahrenheit -- over 15 Celsius -- with a relative humidity of 10% or less. The air is so dry that when it hits a snowpack, the frozen water evaporates, going directly from the ice to vapor and bypassing the liquid phase entirely. This is called sublimation, and it's a common way for snow to disappear in the arid West.
In the East, it's a different story. Even when conditions are dry, they're usually cool enough that the relative humidity doesn't get as low as it does in a Chinook. The way to get rid of an Eastern snowpack all at once is to have several humid days slightly above freezing-which brings the entire snowpack close to melting-then to follow up with a heavy, warm rain. That's what happened to Pennsylvania in January 1996, when over two feet of snow melted in just a couple of days, causing record floods. In that case, a little sublimation could have prevented a lot of consternation.
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