How Unusual Jet Stream Patterns Make Heat Waves Worse

Extreme and prolonged heat waves are becoming more frequent as the global climate warms. Unusually hot conditions across western North America, the Mediterranean and southern Asia this year have been exacerbated by changes in the jet streams. People may know these fast-flowing ribbons of air mainly as the reason it can be quicker to fly west to east than back on the same route. But the location, direction and speed of jet streams determine much of our weather. Scientists expect jet streams may become more unstable in the future, with large implications for our weather systems.

Jet streams are narrow currents of air that circle the Earth, reaching speeds of more than 200 miles (322 kilometers) per hour. There are many jet streams, but when people use the term, they’re usually referring to the four major ones: one each near the north and south poles, and two subtropical ones, closer to the equator. These four, running west to east, encircle the earth at an altitude of about 5 miles (9 kilometers) in the area known as the tropopause — between the lowest level of the earth’s atmosphere (the troposphere) and the cooler, higher level (the stratosphere).