Tree rings have been used as indicators of either temperature or rainfall (actually precipitation, since it may also come in the form of snow), depending on location. In dry areas such as the US Southwest or the Middle East, tree ring widths can match 70% of the variability in measured precipitation for the period of overlap, which is the length of instrumental record (typically about 100 years). When the tree rings are a good match to the instrumental record, they can be used as indicators of past precipitation or drought for centuries before instrumental data are available. The maps above depict Palmer Drought Index, which is a combination of precipitation and temperature, as higher temperatures combined with lower rainfall produce water shortages (drought) which limits plant growth. In high elevation or high latitude locations, tree rings more typically respond to temperature. Statistical matching of ring widths to measured temperature or precipitation is always done, and only trees with strong correlations to the measured data are used in climate reconstructions.