Jet streams are fast-moving currents of air that circulate above the Earth. When people refer to "the jet stream" they are usually referring to the polar-front jet stream or the subtropical jet stream, two major jet streams that shape weather patterns around the world.
Jet streams travel in the tropopause—the area between the troposphere and the stratosphere—at heights of about 8 to 15 kilometers (5 to 9 miles). The strong air currents, which tend to look like wavy, striated rivers when seen on a jet stream map, form when cold air and hot air meet. Their winds blow from west to east at speeds that range from 129 to 225 kilometers per hour (80 to 140 miles per hour), but they can reach more than 443 kilometers per hour (275 miles per hour).
Jet streams are stronger in winter in the northern and southern hemispheres, because that's when air temperature differences that drive them tend to be most pronounced. The polar-front jet stream forms at about 60 degrees latitude in both hemispheres, while the subtropical jet stream forms at about 30 degrees.
How jet streams affect the weather
Shifting jet stream patterns can have a big impact on the weather. Jet streams are always changing: moving to higher or lower altitudes, breaking up, and shifting in flow, depending on the season and other variables, such as energy coming from the sun. During winter, jet streams tend to follow the sun's elevation and move toward the equator, while they move back toward the poles in spring.
Air north of a jet stream is typically colder, while air to the south is usually warmer. As jet streams dip or break off, they move air masses around, creating shifts in global weather patterns. A large buckle in the jet stream, for example, is what pulled Hurricane Sandy ashore in New Jersey in 2012.
Jet streams travel in the tropopause—the area between the troposphere and the stratosphere—at heights of about 5 to 9 miles (8 to 15 kilometers). The strong air currents, which tend to look like wavy, striated rivers when seen on a jet stream map, form when cold air and hot air meet. Their winds blow from west to east at speeds that range from 80 to 140 miles per hour (129 to 225 kilometers per hour), but they can reach more than 275 miles per hour (443 kilometers per hour).
Climate change and the jet stream
Rising global temperatures from global warming are affecting the jet stream and, in turn, the weather. Because the Earth's polar regions are warming more quickly than the rest of the world, the temperature contrast that drives jet streams has decreased. Slower, weaker jet streams have been linked to melting in Greenland and a potential rise in deadly weather events because they can lock weather systems into place, stalling them over regions.
Studies also have linked a warming Arctic with more severe winter weather in the United States, even though other reports note that on average, winter cold snaps are actually getting warmer because of climate change. Part of this link involves the polar vortex, a swirling low-pressure center at the North and South poles. An unstable polar vortex can expand and send cold Arctic air into the jet stream, leading to frigid winter weather and storms southward.