Average

Ring:

Average rings represent average climate water deficit, plant's evaporative demand that exceeds soil moisture during 1950-2000. For every 4% decrease in annual climate water deficit the rings are darker and further apart, for every 4% increase in climate water deficit, the rings are lighter and closer together.   For more info see technical notes.

This data set was only available through 2009.

Here's another index that includes the recent drought.