• about 48 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of August;
• severe drought affected about 33 percent of the contiguous United States as of the end of August 2002, which is a decrease from the 37 percent coverage at the end of July;
• the coverage of the current (November 1999-present) national-scale drought first peaked in August 2000 at about 36% of the contiguous U.S., which was as extensive as the major droughts of the last 40 years, but not as large as the "dust bowl" droughts of the 1930s and 1950s;
• the total drought area decreased to about 10 percent by November 2000, but has shown a general increasing trend for much of the period since then, reaching in June 2002 the peak it had first reached in August 2000 (see graph below left);
• on a broad scale, the last two decades were characterized by unusual wetness with short periods of extensive droughts, whereas the 1930s and 1950s were characterized by prolonged periods of extensive droughts with little wetness (see graph below right);
• although various parts of the U.S. have experienced unusually wet conditions during the last 34 months, little change occurred in the aggregate national wetness picture during much of this period;
• the percentage of the nation severely wet has remained below about eleven percent during this period, reaching about 6 percent by the end of August 2002 (see graph below left);
• a file containing the national monthly percent area severely dry and wet from 1900 to present is available;
• historical temperature, precipitation, and Palmer drought data from 1895 to present for climate divisions, states, and regions in the contiguous U.S. are available at the Climate Division: Temperature-Precipitation-Drought Data page in files having names that start with "drd964x" and ending with "txt" (without the quotes).

The bottom graph to the right shows the percentage of area in the conterminous U.S. influenced by drought since 1700. The graph contains data both from instrumental records and from paleoclimatic reconstructions based on tree-ring analysis. In both cases, the data are percentages of area exhibiting highly negative values of the summer (June-July-August) Palmer Drought Index. The time series in green are percentages of area severely dry in a reconstruction of the Palmer Index using tree ring data covering the period 1700-1978. (The methodology of the tree-ring reconstructed Palmer index is discussed in an article by Cook et al., 1999; details are also available at the NOAA Paleoclimatology web site.) The time series in blue is from the Palmer Hydrological Drought Index computed from June-August weather observations over the period 1900-2002.

Because of the conservative nature of the tree-ring reconstructions, the cut-off value for strong droughts has been adjusted to provide similar ranges between the reconstructed and instrumental data sets. Years with Palmer Index values at or below -2 were considered strong droughts in the reconstructed (tree-ring) record, while those at or below -3 were considered strong droughts in the instrumental record. Noted years are those where more than 30% of the conterminous U.S. was covered by strong drought as revealed from the reconstructed data (green numbers), instrumental data (blue numbers), or both (black numbers).

The tree ring data successfully reproduce the droughts of the early 1910s, mid-1920s, 1930s, and 1950s, although due to its nature it is a conservative index of severity. The 1934 drought was the most widespread national drought in the last 300 years. However, the reconstructed index shows that national droughts nearly as severe as the worst during the 20th century have occurred many times during the past three centuries. More importantly, prolonged severe droughts lasting many years have occurred:

• the 1735-36, 1756-57, and 1863-64 national droughts were each back-to-back summer droughts affecting 28% or more of the conterminous U.S.; back-to-back national droughts of this magnitude didn't happened in the 20th century;
• seven consecutive summers with persistent severe drought occurred during the period 1818-24, with five out of the seven ranking at 20% or higher.

The map to the right above illustrates the areal extent of severe summer dryness during the megadrought of the 1576-1585 period. (The details of this analysis are discussed in an article by Fye et al., 2002).

References:

Cook, E.R., Meko, D.M., Stahle, D.W. and Cleaveland, M.K. 1999. "Drought reconstructions for the continental United States." Journal of Climate, 12:1145-1162.

Stahle, D.W., E.R. Cook, M.K. Cleaveland, M.D. Therrell, D.M. Meko, H.D. Grissino-Mayer, E. Watson, and B.H. Luckman, 2000. "Tree-ring data document 16th century megadrought over North America." Eos, Transactions, Amer. Geophys. Union, 81:121-125.

Fye, F.T., D.W. Stahle, and E.R. Cook, 2002. "Paleoclimatic analogs to 20th century moisture regimes across the USA." Bulletin of the American Meteorological Society, in press.