The researchers are trying to understand the larger spatial patterns and timing of drought in the arid and semiarid areas of the American West.

"We generally understand that the Medieval Climate Anomaly, a warm period in much of the northern hemisphere that occurred about 950 to 1250 was a dry period in the American West," said Byron A. Steinman, postdoctoral researcher in meteorology, Penn State. "But there is complexity to the patterns of drought and it may not have been dry in winter in the Pacific Northwest."

East of the Cascade Mountains, the Pacific Northwest is dry and hot in the summer and wet in the winter now.

Estimates of past precipitation are made from proxies like tree rings, which can record amounts of precipitation and temperature. But tree rings are better at recording what happens during the spring and summer, when the tree is growing, than in the winter when the tree is not.

Steinman, who worked with Mark B. Abbott, professor of geology and planetary science, University of Pittsburgh, his Ph.D. advisor, looked at oxygen isotopes found in 1,500 years of sediment at the bottom of lakes. The isotopic composition of these sediments can reflect the amount of water that enters the lake, especially during the wet season.

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