Reported January 2007

BACKGROUND: Scientists at the National Oceanic and Atmospheric Administration (NOAA) have devised a new, easy-to-understand scale to categorize major snowstorms after they hit the Northeast. The Northeast Snowfall Impact Scale, or NESIS, quickly calculates the impact of a powerful snowstorm soon after it strikes, ranking it similar to methods used to categorize the strength of tornadoes or hurricanes. While winds are used to measure a hurricane's intensity, NESIS will rank the severity of a Northeast snowstorm based on snowfall amounts and the population of the affected areas, then compare it with past storms (dating back to as early as the late 1800s) before assigning it a category: Notable, Significant, Major, Crippling or Extreme.

HOW STORMS DEVELOP: Storm clouds form as moisture evaporates from the earth into the atmosphere, where the droplets congregate and jostle against each other. The air cools rapidly as it rises and the water vapor "condenses" into liquid drops, forming clouds. The process continues: more and more water vapor turns into liquid, and the moist air warms even more and rises higher and higher.

Winter storms are those where, instead of rain, the dominant form of precipitation is snow, sleet or freezing rain; if large amounts of snow fall, it is considered a snowstorm. Add strong winds and "whiteout" conditions, and it might be considered a blizzard. Snow is less dense than liquid water, so the same amount of water that would amount to less than an inch of rain could produce as much as eight inches of snow. Just two inches of snow is sufficient to create traffic disruptions, particularly in cities where snowfall is uncommon. If more than 12 inches of snow accumulate, the weight can cave in the roofs of homes and cause power loss.

THE ICE STORM: Freezing rain storms are especially dangerous. They occur when a layer of warm air hovers over a region where the ground temperature is below freezing. If it results in widespread icing of plants, structures, and roads, it is deemed an ice storm. Even an ice storm with an accumulation of half an inch can paralyze a region. Driving is extremely hazardous, telephone and power lines can be damaged, and in rural areas, crops can be damaged.

The American Meteorological Society contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Carmeyia Gillis
Public Information Officer
National Climatic Data Center
(301) 763-8000 ext. 7163
carmeyia.gillis@noaa.gov

American Meteorological Society
45 Beacon Street
Boston, MA 02108-3693
(617) 227-2425
http://www.ametsoc.org

FACTOIDS...

An ice storm in December 2005 cut across a large portion of the Southern United States, leading to widespread power outages and at least seven deaths.

In 1816, a winter storm hit the Northeast during an abnormally cold summer.

ON THE WEB...

NOAA News Article