Atmospheric Circulation

During World War II, an especially dramatic type of atmospheric air movement was discovered: the jet streams. On a bombing raid over Japan, a sortie of B-29 bombers found themselves being carried along with a tail wind of about 186 MPH (300 km/h). After the war, meteorologists found that these winds were part of permanent air movements now known as the jet streams. Jet streams are currents of air located at altitudes of 30,000–45,000 ft (9,100–13,700 m) that generally move with speeds ranging from about 30–75 MPH (50–120 km/h). It is not uncommon, however, for the speed of jet streams to be much greater than these average figures, as high as 300 MPH (500 km/h) having been measured.

The jet streams discovered in 1944 are formed along the polar front between the Ferrell and polar cells. For this reason, they are usually known as polar jet streams. Polar jet streams usually travel on a west to east direction between 30°N and 50°N latitude. Commercial aircraft often take advantage of the extra push provided by the polar jet stream when they travel from west to east, although the same winds slow down planes going in the opposite direction.

The pathway followed by jet streams is quite variable. They may break apart into two separate streams and then rejoin, or not. They also tend to meander north and south from a central west-east axis. The movement of the jet streams is an important factor in determining weather conditions in mid-latitude regions.

Since the end of World War II, jet streams other than those along the polar front have been discovered. For example, a tropical easterly jet stream has been found to develop during the summer months over Africa, India, and southeast Asia. Some low-level jet streams have also been identified. One of these is located over the Central Plains in the United States, where topographic and climatic conditions favor the development of unusually severe wind systems.