The ribbon of fastest wind in the troposphere
Jet streams are narrow corridors of wind at the boundary between air masses of different temperatures. The strongest jet streams sit at the tropopause where the temperature contrast between equatorial and polar air is sharpest. Wind speeds within the core can exceed 100 m/s. There are typically two subtropical jets and two polar jets per hemisphere, with the polar jets being more variable and stronger during winter when the equator-to-pole temperature gradient is steepest.
The jet streams do not blow straight eastward; they meander in enormous Rossby waves that loop poleward and equatorward. These meanders are crucial for transport of heat from tropics to poles and they steer weather systems. A northward meander over North America brings cold Arctic air southward; an equatorward dip can allow tropical warmth to reach high latitudes.
Steering systems and weather forecasting
The jet stream acts like a conveyor belt for storms. High- and low-pressure systems ride along jet stream meanders, and their speed and direction are largely determined by the local jet stream flow. Strong jets steer systems quickly eastward; weak jets cause weather to stall. This fundamental relationship makes jet stream position and intensity one of the most important inputs to medium-range weather forecasts.
Jet stream variability also contributes to extreme weather events. When the jet stream stalls, a heat dome can linger over a region for weeks, or a trough can park a series of storms in one location. Seasonal changes in jet stream position control the monsoon onset and the transition between seasons. The jet stream is, in essence, the chief conductor of Earth's mid-latitude weather.