Winter is coming—eventually. And while the earth is warming, a new study suggests that the atmosphere is being pushed around in ways that cause long bouts of extreme winter cold or wet in some regions. The study’s authors say they have identified giant meanders in the global jet stream that bring polar air southward, locking in frigid or wet conditions concurrently over much of North America and Europe, often for weeks at a time. Such weather waves, they say, have doubled in frequency since the 1960s. In just the last few years, they have killed hundreds of people and paralyzed energy and transport systems.