It was the summer of 2003 in Europe, and, for a while, it seemed as if Earth’s weather system had broken down. For weeks a huge mass of air stalled over the continent, slowly subsiding and suppressing cloud formation, leaving day after day of brilliantly clear skies. The mercury rose, and a record-breaking heat wave gripped countries including France and Germany, causing 70,000 deaths. Then, as abruptly as it set in, the persistent atmospheric block eased, and high winds brought relief.

Few weather phenomena are as widely experienced—but poorly understood—as an atmospheric block. When a block arises, typically at the western edge of a continent, the jet stream splits, trapping a blob of seemingly static air thousands of kilometers across. Such blocks can last for weeks, and drive heat waves, drought, and winter cold snaps.