Jet stream is a band of fast-moving air currents that occur at high altitudes. Jet streams flow in wavelike fashion around the Northern and Southern hemispheres. They change position constantly and move both vertically and horizontally.
A jet stream’s core of strongest winds is about 60 miles (97 kilometers) wide and about 1 mile (1.6 kilometers) thick. The length of the core varies greatly, but it averages about 3,000 miles (4,800 kilometers). The winds move more than 65 miles (105 kilometers) per hour and may exceed 200 miles (320 kilometers) per hour.
There are three main jet streams in the upper troposphere, the layer of the atmosphere nearest the earth. They occur at altitudes of 6 to 9 miles (10 to 15 kilometers) above the ground. These three jet streams are (1) the polar jet, (2) the subtropical jet, and (3) the equatorial jet.
The polar jet flows from west to east. In the Northern Hemisphere, its position varies so much that it is not shown on the map, but it generally lies between 30° and 60° north latitude (see Latitude). The subtropical jet also travels in an easterly direction. The polar jet and the subtropical jet both weaken during the summer and move farther north. The equatorial jet flows from east to west. Unlike the polar and subtropical jets, it does not circle the earth. It occurs only over Southeast Asia and Africa and only in the summer.
Other jet streams occur at higher altitudes than those found in the troposphere. For example, the polar night jet is in the stratosphere, the atmospheric layer above the troposphere. It moves in an easterly direction and occurs only during the winter.
Jet streams were not widely known until World War II (1939-1945), when American and German pilots encountered them at high altitudes. These strong winds can significantly affect the speed of aircraft flying in or near them. In addition, the force of the winds in and near jet streams may cause aircraft in their vicinity to experience turbulence. Jet streams also influence Earth’s weather. These air currents are often associated with storms and even tornadoes.
