How to rate a snowstorm.

For decades, meteorologists have rated weather phenomena such as tornadoes and hurricanes on a scale of 1 to 5, but they've never really had a good rating system for snowstorms.

A new scale, unveiled on Jan. 30 at the annual meeting of the American Meteorological Society in Atlanta, enables scientists to rank each snowstorm affecting the northeastern United States according to amount of its snowfall, the size of the region it covers, and the population of the affected area. Each storm ranks in one of five categories: notable, significant, major, crippling, or extreme.

The new system is primarily intended to gauge the potential physical and economic impacts of a storm that has just occurred, says Louis Uccellini, director of the National Centers for Environmental Prediction in Camp Springs, Md. The researchers focused on the 13 states of the Northeast because that's where the most significant impact from U.S. snowstorms occurs.

Only two snowstorms in that area in the past century--one in March 1993, the other in January 1996--would have earned an extreme rating, says Uccellini. Such storms affect more than 65 million people and blanket much of a 775,000-square-kilometer area with at least 50 centimeters of snow. Using the new scale, meteorologists rated the blizzard that swept through the Northeast last weekend as a category 3, or major, snowstorm.--S.P.

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