  The mean North American and world climates have warmed significantly since
the beginning of climatologically significant anthropogenic emission of
greenhouse gases in the 19th Century. It has been suggested that warming may
increase the frequency or severity of droughts. We define and study the
statistics of an aridity index that describes the precipitation forcing
function of a drought, considering drought to be a season with low enough
precipitation to be significant for agriculture. Our aridity index is a
reciprocal function of the seasonal precipitation, which is more significant
for agriculture than mean precipitation. Using NOAA data from sites in 13
diverse climate regimes in the 48 contiguous United States with time series
running over the period 1940--1999 but including two data series from 1900 or
1910, and computing their decadal averages, we search for linear trends in
their aridity indices. We find no linear trends significant at the $2\sigma$
level. At five sites $3\sigma$ upper bounds on any systematic trends are in the
range 1.0--2.8%/decade, while at two sites $3\sigma$ lower bounds are
-0.5%/decade and -2.2%/decade; at other sites the bounds are less restrictive.
