Jet stream

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east). Their paths typically have a meandering shape. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet. Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east). Their paths typically have a meandering shape. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet. The strongest jet streams are the polar jets, at 9–12 km (30,000–39,000 ft) above sea level, and the higher altitude and somewhat weaker subtropical jets at 10–16 km (33,000–52,000 ft). The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round. Jet streams are the product of two factors: the atmospheric heating by solar radiation that produces the large-scale Polar, Ferrel, and Hadley circulation cells, and the action of the Coriolis force acting on those moving masses. The Coriolis force is caused by the planet's rotation on its axis. On other planets, internal heat rather than solar heating drives their jet streams. The Polar jet stream forms near the interface of the Polar and Ferrel circulation cells; the subtropical jet forms near the boundary of the Ferrel and Hadley circulation cells. Other jet streams also exist. During the Northern Hemisphere summer, easterly jets can form in tropical regions, typically where dry air encounters more humid air at high altitudes. Low-level jets also are typical of various regions such as the central United States. There are also jet streams in the thermosphere. Meteorologists use the location of some of the jet streams as an aid in weather forecasting. The main commercial relevance of the jet streams is in air travel, as flight time can be dramatically affected by either flying with the flow or against, which results in significant fuel and time cost savings for airlines. Often, the airlines work to fly 'with' the jet stream for this reason. Dynamic North Atlantic Tracks are one example of how airlines and air traffic control work together to accommodate the jet stream and winds aloft that results in the maximum benefit for airlines and other users. Clear-air turbulence, a potential hazard to aircraft passenger safety, is often found in a jet stream's vicinity, but it does not create a substantial alteration on flight times. After the 1883 eruption of the Krakatoa volcano, weather watchers tracked and mapped the effects on the sky over several years. They labelled the phenomenon the 'equatorial smoke stream'. In the 1920s, a Japanese meteorologist, Wasaburo Oishi, detected the jet stream from a site near Mount Fuji. He tracked pilot balloons, also known as pibals (balloons used to determine upper level winds), as they rose into the atmosphere. Oishi's work largely went unnoticed outside Japan because it was published in Esperanto. American pilot Wiley Post, the first man to fly around the world solo in 1933, is often given some credit for discovery of jet streams. Post invented a pressurized suit that let him fly above 6,200 metres (20,300 ft). In the year before his death, Post made several attempts at a high-altitude transcontinental flight, and noticed that at times his ground speed greatly exceeded his air speed.German meteorologist Heinrich Seilkopf is credited with coining a special term, Strahlströmung (literally 'jet current'), for the phenomenon in 1939. Many sources credit real understanding of the nature of jet streams to regular and repeated flight-path traversals during World War II. Flyers consistently noticed westerly tailwinds in excess of 160 km/h (100 mph) in flights, for example, from the US to the UK. Similarly in 1944 a team of American meteorologists in Guam, including Reid Bryson, had enough observations to forecast very high west winds that would slow bombers going to Japan. Polar jet streams are typically located near the 250 hPa (about 1/4 atmosphere) pressure level, or seven to twelve kilometres (23,000 to 39,000 ft) above sea level, while the weaker subtropical jet streams are much higher, between 10 and 16 kilometres (33,000 and 52,000 ft). Jet streams wander laterally dramatically, and have large changes in their altitude. The jet streams form near breaks in the tropopause, at the transitions between the Polar, Ferrel and Hadley circulation cells, and whose circulation, with the Coriolis force acting on those masses, drives the jet streams. The Polar jets, at lower altitude, and often intruding into mid-latitudes, strongly affect weather and aviation. The polar jet stream is most commonly found between latitudes 30° and 60° (closer to 60°), while the subtropical jet streams are located close to latitude 30°. These two jets merge at some locations and times, while at other times they are well separated. The northern Polar jet stream is said to 'follow the sun' as it slowly migrates northward as that hemisphere warms, and southward again as it cools. The width of a jet stream is typically a few hundred kilometres or miles and its vertical thickness often less than five kilometres (16,000 feet). Jet streams are typically continuous over long distances, but discontinuities are common. The path of the jet typically has a meandering shape, and these meanders themselves propagate eastward, at lower speeds than that of the actual wind within the flow. Each large meander, or wave, within the jet stream is known as a Rossby wave (planetary wave). Rossby waves are caused by changes in the Coriolis effect with latitude. Shortwave troughs, are smaller scale waves superimposed on the Rossby waves, with a scale of 1,000 to 4,000 kilometres (600–2,500 mi) long, that move along through the flow pattern around large scale, or longwave, 'ridges' and 'troughs' within Rossby waves. Jet streams can split into two when they encounter an upper-level low, that diverts a portion of the jet stream under its base, while the remainder of the jet moves by to its north.