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There are two main jet streams at polar latitudes, one in each hemisphere, and two lesser streams closer to the equator. In the Northern Hemisphere the streams are most commonly found between latitudes 30°N and 70°N for the polar jet stream, and between latitude 20°N and 50°N for the subtropical. The wind speeds vary according to the temperature gradientMultivariate calculus In vector calculus, gradient is a vector-valued operator that acts on a scalar field. The gradient of a scalar field is a vector field called gradient field which shows its rate and direction of change. For example, consider a room., averagingIn mathematics, there are numerous methods for calculating the average or central tendency of a set of n numbers. The most common method, and the one generally referred to simply as the average is the arithmetic mean. Please see the table of mathematical 55 km/h or 35 mph in summerSummer is one of the four temperate seasons. Astronomically, it begins with the summer solstice (around 21 June in the Northern hemisphere, and 21 December in the Southern hemisphere) and ends with the autumn equinox (around 21 September in the Northern h and 120km/h or 75 mph in winterThis article is about the winter season. For other uses of the term, see winter (disambiguation). Winter is one of the four seasons of temperate zones. Astronomically, it begins with the winter solstice (around December 21 in the Northern hemisphere and J, although speeds of over 400km/h or 250 mph are known. Technically the wind speed has to be higher than 90km/h or 55 mph to be called a jet stream.
The location of the jet stream is an extremely important datum for airlineAn airline is an organization providing aviation services to passengers and/or cargo. It owns or leases airliners with which to supply these services and may form partnerships or alliances with other airlines for reasons of mutual benefit. Airline services. In the United StatesThe United States of America also referred to as the United States U. America ¹ or the States is a federal republic in central North America, stretching from the Atlantic in the east to the Pacific Ocean in the west. It shares land borders with Canada in and Canada, for example, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by the same amount if it must fly west against it. On international flights, the difference is even greater, and it is often actually faster flying eastbound than taking the great circle route between two points.
The jet streams were first discovered during World War II by pilots flying bombers at high altitudes. The first practical use of jet streams was presumably the Japanese fire balloon attacks on the American mainland later during the war.