Trends in mean monthly maximum and minimum surface wind speeds in the coterminous United States, 1961 to 1990

Mean monthly wind speed maxima and minima at 187 and 176 stations (respectively) in the coterminous United States are examined for 1961 to 1990. Both maximum and minimum winds are highest in spring and lowest during the summer. Reduced speeds occur in the western United States, where surface topography is most variable, and in the southeast, which is dominated by high pressure during much of the year. The central and the northeastern United States, which have more gentle topography and are near common storm tracks, have the highest maximum and minimum winds. Least-squares regression lines fit to the 30 yr time series show that, on balance, mean monthly maximum winds are increasing within the United States and mean monthly minima are decreasing. When averaged over all stations, the increase in maximum winds is largest in summer and autumn, with decreases occurring from February through May. Minimum speeds decrease in all months of the year, with the smallest reductions in summer and the largest in spring. In absolute terms the increase in maximum winds is larger than the decrease in minima, but in percentage terms the change in minimum speeds is larger than the change in maxima. Possible causes of the observed trends in maximum and minimum surface wind speeds include hemispheric temperature trends, changes in cyclone and anticyclone frequency, urbanization effects, and instrumentation and observation biases.