Drought variability in North America is of crucial importance not only to farmers but to people throughout the world who rely on food and other materials from the region. Yet instrumented records provide only a limited picture of the occurrence of and factors impacting drought variability in the North American continent.

The drought data are in the form of summer (June-July-August) average Palmer Drought Severity Index. Negative PDSI values indicate dry conditions, while positive values indicate wet conditions. PDSI values generally fall between -6 and +6.

In 1999, a gridded network of PDSI reconstructions for the continental United States were generated from a set of 388 tree-ring chronologies. Data are displayed in the form of annual contour maps, and in graphed time series for individual grid points. Maps and data from graphs can be downloaded, as well as the entire set of instrumental and reconstructed data.

In August 2004, the spatial and temporal coverage of the PDSI reconstructions was expanded. The new grid includes 286 points in a 2.5 degree grid covering most of North America. The PDSI reconstructions are based on an expanded network of 602 tree-ring chronologies. The temporal coverage has been expanded to the maximum permitted by the available tree ring data, extending back nearly 2000 years for some locations. The instrumental PDSI series have been extended up through 2003. The new reconstructions are currently available from this site in numerical form and as time series plots; the mapping display tools are being updated to display the expanded network.

Additional information on drought can be found in these recent publications.

For additional information, see our Paleo Perspective on North American Drought

These pages were developed jointly by the NOAA Paleoclimatology Program, the National Geophysical Data Center and the National Climatic Data Center. The 2004 reconstructions were funded by the Earth System History (ESH) of the National Science Foundation in a grant to E.R. Cook.