How do we tell how much snow fell in a storm?
How do we tell how much snow fell in a storm?
The monster “bomb cyclone” that hammered the US East Coast this week made it snow in places that are rarely coated in white: Savannah, Georgia got some 1.2 inches of snow, making records; while Charleston, South Carolina got 5.3 inches on January 3rd, the city’s third-snowiest day on record. So how is snowfall measured exactly? It’s trickier than you might think — and it’s still largely done manually with pretty rudimentary equipment.When snow falls, it’s blown by the wind all over the place, and once it’s fallen, snow tends to compact under its own weight — or melt if the Sun starts shining. So the US National Weather Service actually has pretty precise guidelines for how to measure snow. These guidelines are followed by so-called observers at airports and weather forecast offices, as well as by the nearly 9,000 (trained) volunteers who take measurements daily in farms, towns, cities, and national parks all across the US.“It’s a tricky measurement because there are all these uncertainties associated with how the wind blows the snow around and the variations in densities, tied even to the shape of the snow crystal,” says Justin Minder, assistant professor of atmospheric and environmental sciences at University at Albany. “It makes it a fascinating thing to study, although it’s challenging.”Other than a measuring stick, which is basically a ruler with measures of tenths of an inch, observers need a snow measuring board, also called a snowboard. This is nothing fancy: a flat 24-inch x 24-inch piece of plywood that’s painted white, so that it doesn’t heat up with sunlight that would melt the snow. The snowboard needs to be placed in an area that’s somehow protected from the wind and shaded from the Sun, but also far enough away from buildings or trees that might shield the snow. Every six hours, observers need to check that snowboard and measure how much snow has fallen, then wipe the snowboard clean to measure snowfall for the next six hours. At the end of the 24-hour period, the measurements are added up to get a total snowfall for that day. That mostly happens at airports or weather stations, obviously. The NWS understands that volunteers have a life — and asks them to take measurements only once a day. (Ideally, the measurements are taken right after the snow stops falling, so it doesn’t settle or melt.) “Sounds easy, but sometimes the precipitation type changes and if it rains on top of the snow, you have to get the snow measurement before the snow melts or crushes it down,” says Bill Syrett, who has been managing the weather station at Penn State for over 20 years. “An observer has to keep his or her eyes open to what’s going on.”The NWS also asks its observers to measure the accumulation of all the snow that’s on the ground, which is done by using a measuring stick in three or more locations, and then averaging those numbers out. And it wants to know how much water is inside all the snow that’s freshly fallen, or accumulated on the ground. To do that, observers have to actually melt a snow sample collected from the snowboard or a rain gauge. In fact, snow can be dry and fluffy, or wet and heavy — and knowing how much water is in it is key for calculating flood risk once that snow melts, or knowing how much water is going to be added to reservoirs and rivers used for drinking water and irrigation. Some weather stations around the country, like the New York State Mesonet, are using automated sensors that measure snow depth every five minutes, says Minder. But these sensors can’t work if it’s snowing too heavily. So the manual measurements are still the most trusted, especially when done by weather service personnel or at airports, Minder says. But even the volunteers receive training, and although measuring snowfall is tricky, it’s doable.“At first, it does confuse some people, but once you start doing it, it all makes sense” and takes around 15 minutes a day, says James Zdrojewski, a program analyst at the NWS who helped write the NWS Snow Measuring Guidelines and used to train volunteers in Oregon. Plus, the volunteers often “become their own local celebrity ... as the town weatherperson,” he says. For meteorologists like Syrett, it’s not about being a weather celebrity, but about an unconditional love for storms. He says he’s been fascinated by them since he was at least five. “I love it when I get to experience thunderstorms, snowstorms, all that exciting stuff.” That’s why he’s pretty disappointed by the “boring winter” he’s had so far: not a lot of snow has fallen at the Penn State weather center in central Pennsylvania. I, on the other end, spent Christmas in Erie in northern Pennsylvania, where a so-called lake-effect snowstorm dumped more than five feet of snow, shattering records. I then came back to New York City, when the bomb cyclone brought more snow and frigid temperatures. Enough winter for me!