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During her summer travels to her native Belgium, University of Arizona professor Valerie Trouet noticed something that turned casual curiosity into a major scientific discovery: when the sun hid behind an overcast sky and people around her put on sweaters instead of summer clothes, the weather tended to be warm and dry in Italy, Greece and the Balkans, popular summer escapes for tourists from the cooler climates of central and northern Europe.

At U of A's Laboratory of Tree-Ring Research, Trouet studies tree-rings to gather clues about what past climates were like, reading wavy, wooden lines like a linguist might decipher an ancient text. What if, she mused, the key to understanding the capricious summers in Europe could be hidden in trees, silent witnesses to centuries of warm and cold, sunshine, rain and snow?

Trouet assembled an international collaboration to collect tree-ring samples across Europe. The team published its results—the first reconstruction of the jet stream over the past 700 years—in the journal Nature.

The jet stream and the Black Death

Jet streams are concentrated bands of wind in the upper atmosphere that travel around the globe in the northern and southern hemispheres. Their exact locations are not fixed; in response to changes in the position and intensity of high- and low-pressure weather systems, they may shift north or south or change their course, resembling a swiftly running stream at some times, and a slow, meandering river at others.

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