Snow impacts activities as diverse as engineering, agriculture, travel, recreation, commerce, and safety. For instance, an excessive amount of water held in a regional snow pack may lead to spring flooding, while conversely, a meager snowpack may result in subsequent water shortages for drinking, industry and agriculture. Negative departures during the recent prolonged period of low extent have been most pronounced in spring, and winter cover has shown the least variation. The reduced snow extent in recent years has coincided with some of the warmest temperatures of the past century. A striking relationship between hemispheric snow extent and surface air temperature has been observed over the past two decades.
Correlations are strongest over North America and western Europe.
In 1966, NOAA began to map snow areas in the Northern Hemisphere on a weekly basis. Charts are based on a visual interpretation of photographic copies of shortwave imagery by trained meteorologists. Most studies of snow extent use monthly values computed from weekly totals. An undocumented change in the procedure used by NOAA to calculate monthly extent occurred in 1981. At Rutgers University, a new, consistent methodology was developed that calculates weekly areas and weights them according to the number of days of a chart week falling in the given month.