Description
Polar jet streams are typically located near the 250 hPa pressure level, or 7 to 12 kilometres (4.3 to 7.5 mi) above sea level, while the weaker subtropical jet streams are much higher, between 10 and 16 kilometres (6.2 and 9.9 mi) above sea level. In each hemisphere, both upper-level jet streams form near breaks in the tropopause, that is at a higher altitude near the equator than it is over the poles, with large changes in its height occurring near the location of the jet stream. The northern hemisphere polar jet stream is most commonly found between latitudes 30°N and 60°N, while the northern subtropical jet stream located close to latitude 30°N. The upper level jet stream is said to "follow the sun" as it moves northward during the warm season, or late spring and summer, and southward during the cold season, or autumn and winter.
The width of a jet stream is typically a few hundred kilometres or miles and its vertical thickness often less than five kilometres (3 mi).
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 east, 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. Rossby waves are caused by changes in the Coriolis effect with latitude. Shortwave troughs are smaller packets of upper level energy, on the scale of 1,000 to 4,000 kilometres (620–2,500 mi) long, that move through the flow pattern around large scale, or longwave, ridges and troughs within Rossby waves. Jet streams can split into two due to the formation of an upper-level closed low, that diverts a portion of the jet stream under its base, while the remainder of the jet moves by to its north.
The wind speeds vary according to the temperature gradient, exceeding 92 km/h (50 kn; 57 mph), although speeds of over 398 km/h (215 kn; 247 mph) have been measured. Meteorologists now understand that the path of jet streams steers cyclonic storm systems at lower levels in the atmosphere, and so knowledge of their course has become an important part of weather forecasting. For example, in 2007 and 2012, Britain experienced severe flooding as a result of the polar jet staying south for the summer.
The polar and subtropical jets merge at some locations and times, while at other times they are well separated.
Read more about this topic: Jet Stream
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