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Friday, May 25, 2018

The Jet Stream

     Jet streams are rivers of wind high above in the atmosphere. These narrow strips of strong winds have a huge influence on climate because they can push air masses around and affect weather patterns. The Earth is not the only planet that has jet streams; Jupiter and Saturn also have them. Jet streams are usually found at 20,000 feet (6,100 meters) to 50,000 feet (9,144 meters), or about 7 miles (11 kilometers) above the surface. 
     Observers of the 1883 eruption of the Krakatoa volcanic island in Indonesia documented its effect on the sky and in the 1920s Japanese meteorologist Wasaburo Oishi used aviator balloons to identify the jet stream from a site near Mt. Fuji. German meteorologist H. Seilkopf is often credited with coining the phrase "jet stream," as he used in a research paper published in 1939. 
     Aviation played a role in the discovery and mapping of jet streams. Many credit bomber pilots during World War II with discovering much of the knowledge about the jet streams when they were able to take advantage of jet steams. Scientists had theorized the existence of jet streams at least as early as 1937, but it was for US bomber pilots to confirm it. They were able to speed up their missions over the Mediterranean by making the most of the jet streams. 
     In November, 1944, the Japanese launched unmanned bomb-carrying balloons which they believed would ride the jet stream across the Pacific to North America. More information 
     World War II was almost over when the US introduced the Boeing B-29 high-altitude bomber, which flew at at altitudes well above 22,000 ft. When the B-29s were being put into service from a Pacific island base, two Air Force meteorologists were assigned to prepare wind forecasts for aircraft operations at such altitudes. 
    Using surface observations military meteorologists had predicted a 193 mile per hour (168-knot wind) blowing from the west, but commanding officer did not believe it, but the next day B-29 pilots reported wind speeds of 170 knots from the west. 
     While they are fairly narrow, they cover a wide north to south distance and often travel a very winding path; at times they can even fade away or break off into smaller rivers” of air that merge again downstream. They are affected by factors such as the season, the location of low and high pressure systems and air temperature. Jet streams form a border between hot and cold air. Because air temperature influences jet streams, they are more active in the winter when there are wider ranges of temperatures between the Arctic and tropic air masses. 
     Temperature also influences the speed; the greater the difference in air temperature, the faster the jet stream, which can reach speeds in excess of 250 mph (402 kph), but they average about 110 mph (177 kph). 

    Both hemispheres have jet streams, although the jet streams in the north are more forceful. Each hemisphere has two primary jet streams — a polar and a subtropical. The polar and subtropical jet streams are the best known and most studied, other streams can form when wind speeds are above 58 mph (93.3 kph) in the upper atmosphere at about 6 miles (9.6 kilometers) to 9 miles (14.5 kilometers) above the surface. However, all strong upper-atmosphere winds are not jet streams. 
     Jets streams affect the weather because they usually separate colder air and warmer air causing air masses push air masses around. Jet streams don’t generally follow a straight, but shift which can play havoc with the weather forecasters' predictions. 
     Jet streams also have an impact on air travel and are used to determine flight patterns. An airplane can travel much faster, and save fuel, by flying in the jet stream, but doing so can also result in a bumpy flight.
     Watch the wings of a Boeing 787 shake as the plane flies through clear air turbulence caused by the jet stream. Pilots refer to what is shown here as light chop. One pilot noted that the 787's wings can flex an arc of 30 feet.

How does the jet stream affect flight time? 
How the jet stream affects an airplane ride?
New theory finds 'traffic jams' in jet stream cause abnormal weather patterns 
Jet stream/wind maps

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