After this weekend, snowstorm people are going to be asking, as they always do: What town got the most snow?
The National Weather Service tries to assuage our curiosity with a co-op program that sprinkles trained volunteers throughout the region who supplement official observations made at places like Concord airport, where the official state data is taken, and meteorological stations like the Mount Washington Observatory.
These folks, in places as obscure as Bearcamp River and the unincorporated community of Pike or as urban as Manchester and Nashua, follow a protocol to measure the depth of snow. They maintain a clean, flat, level surface that is not too close to buildings or trees, which might deflect snowfall, and each morning at a set time they measure how much snow has accumulated in the past 24 hours, even if the snow still is falling. Then they clean it off and start over.
In recent years, their data has been supplemented by an online program called Community Collaborative Rain Hail Snow, or CoCoRaHS, run by Colorado State University. It now has more than 10,000 volunteer observers throughout the U.S., including scores in New Hampshire, who measure the past 24 hours’ worth of snow each morning. Past data from about 100 New Hampshire observers can be seen online at the National Weather Service website, or you can search the CoCoRaHS site itself.
So town-by-town snowfall data received from official weather sources is pretty good? Well, maybe.
Speaking as a multiyear CoCoRaHS observer, I can tell you that the whole concept of depth of snow is not very exact. Wind blows snow around, piling it into mounds or scouring it away; trees grab it and hold it, then let go all at once; and small changes in temperature or humidity can alter the size of the snowflakes and change the depth of snow by 50 percent or more.
There’s also the question of where to put your “snow board,” the flat place you measure on, in order to reflect what is happening all around.
I talked about this issue two year ago with James Brown, a hydrometeorological technician with the National Weather Service in Gray, Maine, that covers most of New Hampshire. Because of the government shutdown, nobody at the NWS was available to revisit the topic this week, but Brown’s comments still hold true.
“(The board) is supposed to be twice the distance from the highest object, but that’s hard to do in New England. We have a lot of trees,” Brown said at the time.
So my snow board should be 100 feet from the nearest 50-foot maple tree, but I don’t have that much open space in my property; I can’t follow that recommendation. And if I put the board in the middle of my neighbor’s big field, wind might blow it clean, giving me a measurement of zero even when huge drifts are all around.
In other words, measuring snow depth is not an exact science. “It’s a difficult thing. You have to use your head about it,” is how Brown put it.
Meteorologists don’t really care about the depth of snow. They care about how much moisture is in the snow because that is important for predicting floods or droughts. Depth alone won’t tell you moisture because, as we all know, snow can be wet and heavy or it can be “dry” and fluffy.
The real data point for meteorologists is “snow-water equivalence,” in which a set amount of snow gets melted, and the resulting water is measured as if it were rainfall. That’s got virtually nothing to do with depth.
As for answering the question “exactly how much snow fell in my yard?” — relax. It’s virtually impossible to say precisely, so put the yardstick somewhere obvious and consider the result an intelligent estimate.
Then go play in the snow, because that’s the best way to judge how big this storm was.