The Jet Stream map shows today's high wind speed levels and jetstream directions. Jet streams are fast flowing, relatively narrow air currents found in the atmosphere around 10 kilometers above the surface of the Earth. They form at the boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air to the south. The jet stream is mainly found in the tropopause, at the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height).

Our weather is mainly controlled by the Subtropical Jet Stream. The subtropical jet forms at the poleward limit of the tropical Hadley cell and to first order this circulation is symmetric with respect to longitude. Tropical air rises to the tropopause, mainly because of thunderstorm systems in the Intertropical Convergence Zone, and moves poleward before sinking; this is the Hadley circulation. As it does so it tends to conserve angular momentum, since friction is slight above the ground. In the northern hemisphere motions are deflected to the right by the Coriolis force, which for poleward (northward) moving air implies an increased eastward component of the winds (note that leftward deflection in the southern hemisphere also leads to eastward motion). Around 30 degrees from the equator the jet wind speeds have become strong enough that were the jet to extend further polewards the increased windspeed would be unstable; thus the jet is limited.