Jet streams are very fast moving currents of air found at about 10 km (6.2 miles) above the surface of the earth in the tropopause, which is the transition layer between the Earth’s troposphere and the stratosphere. The major jet streams usually follow an undulating narrow path from west to east. There are jet streams on both sides of the equator. Jet streams close to the poles are called polar jet streams and those at the tropics are known as subtropical jet streams, which are weaker than the polar jets. Jet streams form due to the orbital rotation of the planet along with atmospheric variations in temperature, which produces strong winds.
How were jet streams discovered?
Even though jet streams had existed on Earth for a long period of time before their actual discovery, they were not referred to as jet streams; instead, weather watchers used other terms to refer to them, such as equatorial smoke stream or strahlstromung, which means jet streaming in German. An understanding of the nature of jet streams occurred during regular flights across the Atlantic, when pilots noticed strong winds exceeding 160 km/h (100 mph) in their flying path, although their real existence was first confirmed after the Japanese attempted to drop bombs into the U.S. by sending balloons loaded with explosives, using the strong winds of a jet stream as a flying aid.
Formation of jet streams
Most jet streams flow in a west to east direction in a region located between the troposphere and the stratosphere known as the tropopause, at elevations ranging from 10-14 km (6.2-8.6 miles). Jet streams form in the same way as all winds do, from horizontal variations in air pressure. When two air masses of different temperatures and densities meet, the pressure differences cause strong winds at the point of convergence. The winds do not mix, but flow along the boundaries of the two masses. The polar jet stream usually forms along a polar cold front, where contrasts in temperature cause horizontal pressure changes and strong winds. The eastward direction of the jet streams derives from temperature differences between the equator and the poles.
Kinds of jet streams
There are two types of major jet streams, the polar jet streams and the subtropical jet streams. Polar jet streams typically form from 7-12 km (4.3-7.4 miles) above sea level, while subtropical jet streams, which are weaker, form from 10-16 km (6.2-10 miles) above sea level. The northern polar jet stream usually forms between 30° N and 60° N latitudes, and the subtropical jet stream forms near 30° N latitude. These jet streams usually extend for several hundreds of miles long and less than 5 km (3 miles) thick. Sometimes, the polar jet stream may split into two jet streams, called the northern branch to the north and the southern branch to the south.
How jet streams affect weather
Often, meteorologists utilize the position of some jet streams to forecast weather. The polar jet stream, in the northern hemisphere, is the most utilized as it is stronger and at a lower latitude than the subtropical jet stream. The southern wavy motion of the polar jet stream carries cold air toward the equator, while the northern wavy motion carries hot air toward the pole; thus they contribute to the transferring of heat around the world. The meandering motion of jet streams carries thousands of pollutants and volcanic ash globally. During the winter, the polar jet stream winds are stronger and move farther south, while in the summer they are weaker and form at higher latitudes.
How are jet streams used in aviation?
Knowing the exact location of a jet stream is extremely important for commercial aviation. The time spent on a flight can be cut off by about one-third when flying inside the path of a jet stream. A Pan Am airplane flew from Tokyo to Honolulu, in 1952, in 11 and a half hours from the 18 hours that it would have taken the plane if it had flown outside of the jet stream. Flying in this way, not only reduces the flying time, but it also cuts down fuel costs. In North America, it is very common to fly using the polar jet stream. Clear air turbulence, which is caused by a vertical and horizontal wind cut in jet streams, can cause the plunging of aircraft, leading to fatal accidents.
The friction of air in the Earth’s atmosphere caused by the rotation of the Earth produces various wind patterns which flow along the boundaries of different types of air masses. Cold temperatures at the poles influence the creation of polar jet streams on both hemispheres, and the warmer regions at the equator produce the subtropical jet streams. According to weatheronline.co.uk, meteorologists flying on a Boeing B-29, which could fly at altitudes above 7 km (4.3 miles), confirmed the existence of the jet stream by using surface observations of the thermal wind.