Update, Dec. 27, 2010: Twenty inches of snow fell in New York City on Sunday and Monday, while 31 inches accumulated in parts of New Jersey. During a blizzard in 2003, Brendan I. Koerner wondered how the weather service takes measure of all that snowfall. The article is reprinted below.
The blizzard that's pounding the East Coast is supposedly the worst since 1996 in terms of snowfall. How exactly do meteorologists figure out how much of the white stuff tumbled down?
Science may have unlocked the secrets of the atom and sent probes past Jupiter, but it has yet to devise a foolproof, automated means of measuring snowfall. The most reliable technique still involves a human and a special ruler, divided into tenths-of-an-inch. Before a snowfall, an observer typically lays out several snowboards—not of the recreational variety, but rather simple 16-inch-by-16-inch planks, which are marked with bright flags. Ideally, these boards should be located in wide open areas where drifting will be minimal. The National Weather Service recommends that every six hours the observer should plunge his or her ruler into the snow that's accumulated atop the various boards; after a measurement is taken, each board should be dug up and placed on the freshest layer of snow. Average together the measurements taken from a dozen or so boards, and you've got your snowfall estimate.
The ruler method isn't quite perfect. Some snow may melt in between measurements, and strong winds can cause excessive drifting or blowing, even in open terrain. Plus, it's often hard to determine whether there's any frozen rain in the mix—the NWS frowns upon adding frozen rain totals to snowfall estimates. Still, trudging out with a ruler is a good deal more accurate than the alternate technique, whereby snow is collected in a cylindrical gauge and then melted. The general guideline is that 10 inches of snow is equal to 1 inch of water. This approach is favored for remote spots where observers may not be around to take manual measurements every day; the gauge is simply lined with antifreeze, to hasten melting, and can potentially be monitored from afar.
The problem is that the 10-to-1 ratio is merely a rough guesstimate. The actual water content varies widely, depending on the character of the snow. For the slushy stuff common to urban areas, the content may be more along the lines of 6 inches of snow per inch of water; the driest powder, by contrast, can pack 50 inches of snow into the same space. Unless you're on the scene during the snowfall, it's virtually impossible to tell exactly what varieties of snow fell during the blizzard, and in what proportions.