Biases and large-scale inhomogeneities in the time series of measured precipitation and snowfall over the continent are discussed and analyzed. After adjustments and selection of the ''best" network, reliable "first guess" estimates of North American snowfall and precipitation have been obtained. Century-long time series of unbiased annual precipitation over the regions to the south of 55°N, and 40-year time series of unbiased area-averaged annual precipitation and snowfall for all of North America, have been developed. Analysis of the trends shows that:
During the last hundred years, annual precipitation has increased in southern Canada (south of 55°N) by 13 percent and in the contiguous United States by 4 percent to 5 percent; the main domain of this century-scale precipitation increase, however, is eastern Canada and the adjacent northern regions of the United States.
An increase of up to 20 percent increase in annual snowfall and rainfall has occurred during the last four decades in Canada north of 55°N.
During the last century, global surface air temperature has increased approximately 0.5°C (over Canada it has been about 1°C); the seven warmest years in the instrumental records of global temperature occurred during the last decade (Budyko and Izrael, 1987; Gullett and Skinner, 1992; IPCC, 1990, 1992). With such a large change in the earth's heat balance, we suspect that changes in the water balance of North America may have also occurred. Given the potential for even greater changes over the next decades, there is an urgent need to obtain reliable results on North American precipitation changes during the last century.
Numerous studies have been made of precipitation over North America and the problems associated with precipitation measurement. The results of these studies can be summarized by saying that no definite century-scale precipitation trends have been found for the United States. (Canadian precipitation cannot be analyzed for the entire century,
State Hydrological Institute, St. Petersburg, Russia; current address: Department of Geosciences, University of Massachusetts, Amherst, Massachusetts
NOAA National Climatic Data Center, Asheville, North Carolina