Paleo Slide Set: Tree Rings: Ancient Chronicles of Environmental Change
Reconstructed Palmer Drought Severity Index for 1863, from Cook et al, 1997

Climate reconstructed from tree rings can show both spatial and temporal 
variations in climate. Cook et al. (1997) reconstructed periods of drought 
for a set of grid points across the United States, from 1700 to the present. 
For each year, the distribution of drought severity was mapped, 
using the Palmer Drought Severity Index (PDSI). This slide shows areas 
of drought in the year 1863, a very dry year for the northern Great Plains. 
The extremely dry areas are shown in red, areas of average rainfall in white, 
and areas with very wet conditions are blue.

Photo Credits:
Ed Gille
NOAA Paleoclimatology Program