JET STREAMS — LEE 295 



middle latitudes of each hemisphere. Air motion is generally from 

 west to east ; however, on any individual day, a jet stream may follow 

 a meandering course that dips in some regions into the Tropics and 

 extends north of the Arctic Circle in others. A schematic diagram 

 showing a single jet stream is presented in figure 1. The heavy con- 

 tinuous line defines the axis of the jet stream along which the wind 

 speed attains its maximum values in the horizontal. One can usually 

 find the axis of a jet stream encircling the globe on any given occasion. 



Figure 2 shows a view of a jet stream as seen by an observer look- 

 ing downstream from a point along the axis. The numbers along the 

 bottom of the diagram are the International Station Numbers which 

 identify five stations in Alaska and one in the Yukon, lying approxi- 

 mately in a line oriented from northwest to southeast. From right 

 to left, they are named, respectively, Kotzebu (133), McGrath (231), 

 Fairbanks (261), Big Delta (263), Northway (291), and Whitehorse 

 (964). The distance between Kotzebu and Whitehorse is 735 nau- 

 tical miles. The ordinate is pressure in millibars (mb.) plotted on 

 a logarithmic scale; 500 mb. corresponds very nearly to 18,000 feet, 

 200 mb. to 39,000 feet, and 100 mb. to 53,000 feet. Lines of equal 

 wind speed in knots, called isotachs, are used to portray the wind field. 

 Thus, within the central closed isotach around the main jet axis, 

 labeled J, above 400 mb., the wind speed is in excess of 90 knots. 



If we consider the horizontal width of that band of winds in ex- 

 cess of a given value, say 80 knots, we would find it to be surprisingly 

 narrow — of the order of 100 miles in this example, but generally about 

 300 nautical miles. The vertical depth of the winds greater than 80 

 knots in figure 2 is less than 2 miles. A comparison of the horizontal 

 width of this jet core with the depth would lead us to the conclusion 

 that the jet stream can be represented fairly accurately in shape by 

 a flat ribbon parallel to the earth's surface. Other features on the 

 cross section are the tropopause, indicated by the discontinuous heavy 

 line around the 300-400-mb. levels, and the continental arctic frontal 

 surface separating the relatively warm maritime arctic air mass on the 

 right of the diagram from the cold continental arctic air to its left. 

 The broken lines are isotherms labeled in degrees Centigrade. 



RELATIONSHIP BETWEEN JET STREAMS AND FRONTS 



This particular cross section is typical of the northernmost jet 

 stream which has been encountered by K. C. A. F. flights many times 

 in the past. Further studies of jet streams have revealed that, on the 

 average, four main tropospheric jet streams are present over North 

 America during the winter months. Except for the southernmost 

 subtropical jet stream which usually appears in the vicinity of Florida 

 and Cuba, each of the other three is closely associated with one of 



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