Given our current dry situation, I thought I'd introduce one of the most widely used measures of drought, the Palmer Drought Severity Index, or PDSI.

A wide variety of weather data is used to compute the PDSI, including precipitation, temperature, soil moisture, runoff, and whether or not there's a snowpack. This information is assimilated into one number that measures an area's long-term moisture situation. The PDSI can be used to compare different locations and different times, so it's useful for placing conditions of extreme dryness in a historical perspective.

The way the PDSI is set up, values of -2 to -3 indicate a moderate drought, as shown by yellow on this map of the eastern U.S. -3 to -4 is a severe drought, shown here as light orange, while -4 or less is extreme drought, here shaded a burnt orange. As you can see, all of Delaware and eastern Maryland, and the southern 2/3 of New Jersey, is in an extreme drought, while southeastern Pennsylvania is in a severe drought. But we're not the only ones - expanding our view nationally, we see that nearly the entire western half of the country is also facing severe to extreme drought conditions.

The most recent national map of the PDSI can be found here.