Hourly precipitation data from 1963 to 1993 of the contiguous United States
were analyzed for secular trends in the diurnal cycle of precipitation. Du
ring the period the winter daytime and nighttime precipitation frequencies
and the diurnal amplitude of precipitation frequency increased by similar t
o 30-47% over the Southwest, while the winter precipitation frequencies and
the diurnal amplitude of precipitation intensity decreased by similar amou
nts over the Northwest. During this period, summer afternoon precipitation
frequency increased by similar to 30-60% in the Southwest and decreased by
similar to 15-30% in the Southeast.