Jet streams are fast narrow wind currents occurring at the tropopause, which is the region in the atmosphere between the troposphere and stratosphere, at about 10-14 km (6.2-8.6 miles) of altitude. Jet stream winds flow in a west to east direction at speeds ranging from 100-200 km/h (62-124 miles/h). There are two main types of jet streams, known as the polar jet stream and the subtropical jet stream, which form on both sides of the equator, although the polar jet stream may form other jet streams. Jet streams are produced by the orbital rotation of Earth and the variations in temperature in the Earth’s atmosphere.
Discovery of jet streams
For a long time, ground observations of rapidly moving cirrus clouds had revealed the existence of fast-moving winds high up in the sky. Although the existence of fast-moving winds was suspected, it was not until World War II that their presence was confirmed during regular military flights across the Atlantic Ocean. During this time, pilots flying at high altitudes noticed high-speed winds exceeding 160 km/h (100 miles/h) along their flying route. It was also during this time that the real existence of jet streams was confirmed when the Japanese, in an attempt to drop bombs in U.S. territory, used the jet streams to send balloons with explosives across the Pacific.
Jet stream formation
Jet streams flow from a west to east direction in a region in the atmosphere between the troposphere and the stratosphere called the tropopause, at altitudes ranging from 10-14 km (6.2-8.6 miles) above sea level. The formation of jet streams follows a similar pattern to that of all winds. Variations in air pressure cause air of different masses to displace in the atmosphere. The differences in pressure produce strong winds at the point where two masses of air meet, causing a strong flow of air along the boundaries of both air masses. The polar jet stream is generally found along a cold front of polar air, where differences in temperature produce air pressure variations and very strong winds. The eastward direction of jet streams is due to temperature variations at the equator and the poles.
Kinds of jet streams
The main types of jet streams are the polar jet stream and the subtropical jet stream. Polar jet streams generally form at an altitude ranging from 7-12 km (4.3-7.4 miles) above sea level, while the weaker subtropical jet streams form higher in the atmosphere at 10-16 km (6.2-10 miles) above sea level. The polar jet stream usually forms between 30° N and 60° N latitude, while the subtropical jet stream usually forms near 30° N latitude. Jet streams can be several hundreds of miles long and typically less than 5 km (3 miles) across. The polar jet stream may split into two jet streams, one to the north and another to the south, known as the north branch and the south branch, respectively.
Jet streams and weather
Frequently, meteorologists make use of the jet streams, especially the polar jet stream, to forecast the weather. The polar jet stream, which is the most used, is situated at lower latitude and is stronger than the subtropical jet stream. The polar jet stream carries air to both the north and south. The southern undulating motion of the polar jet stream transports cold air fronts toward the equator, while the northern undulating portion moves hot air toward the pole. This wavy motion distributes heat globally. Besides this, the undulation of jet streams carries pollutants and other particles, such as volcanic ash. The polar jet streams are stronger in the winter, moving farther to the south, while in the summer, they’re weaker and move farther to the north.
The location of jet streams is very important in commercial aviation. The time in flight of an airplane can be significantly reduced by knowing the location of a jet stream. Airplanes have flown inside the path of a jet stream, reducing the flying time by about one-third. Flying along the path of a jet stream reduces flying time and cuts fuel costs. Clear air turbulence is a phenomenon which produces wind cuts in jet streams and can cause serious air incidents. According to encyclopedia.com, wind currents in a jet stream move at different speeds, but the wind reaches its highest speed at the center, where wind speed can reach 322 km (200 miles) per hour.