THE ACCURACY OF UNITED-STATES PRECIPITATION DATA

Precipitation measurements in the United States (as well as all other countries) are adversely affected by the gauge undercatch bias of poin t precipitation measurements. When these measurements are used to obta in areal averages, particularly in mountainous terrain, additional bia ses may be introduced because most stations ate at lower elevations in exposed sites. Gauge measurements tend to be underestimates of the tr ue precipitation, largely because of wind-induced turbulence at the ga uge orifice and wetting losses on the internal walls of the gauge. The se are not trivial as monthly estimates of this bias often vary from 5 % to 40%. Biases are larger in winter than in summer and increase to t he north in the United States due largely to the deleterious effect of the wind on snowfall. Simple spatial averaging of data from existing networks does not provide an accurate evaluation of the area-mean prec ipitation over mountainous terrain (e.g., over much of the western Uni ted States) since most stations are located at low elevations. This te nds to underestimate area averages since, in mountainous terrain, prec ipitation generally increases with elevation. Temporal precipitation t rends for the United States, as well as seasonal and annual averages, are presented. Estimates of unbiased (or less biased) precipitation ov er the northern Great Plains provide a regional analysis.