In the messy, chaotic atmosphere of our planet meanders the jet stream, a wiggly belt of air circling the mid-latitudes. As the belt moves south, it pulls cool air from the Arctic toward the tropics. Then it switches direction, pulling warm air from the tropics toward the poles. Sometimes, in response to natural climate patterns, the jet stream becomes abnormally wavy. Such amplified waves have coincided with heat and cold waves, droughts and flooding across the world, according to a study published yesterday in Nature Climate Change.