298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



the three main frontal surfaces over North America in winter. These 

 three frontal surfaces are respectively called the polar front, the mari- 

 time arctic front, and the continental arctic front, found in this order 

 from south to north. The polar and maritime arctic jet streams have 

 structures very similar to the continental arctic jet stream in figure 2. 

 There is one fundamental difference between them, namely, the 

 height of maximum wind speed is found at higher altitudes as one 

 proceeds southward. For instance, the axis of the continental arctic 

 jet stream is normally found between 25 and 30 thousand feet, the 

 maritime arctic jet stream between 32 and 36 thousand feet, and the 

 polar front jet stream between 35 and 40 thousand feet. These jet 

 streams are also found over Japan in winter. Thus we can see why 

 the strong winds were not encountered by the high-altitude bombers 

 of the Second World War until the scene of operations moved suffi- 

 ciently far north in the western Pacific. 



Another notable fact about the three northernmost jet streams is 

 that the axis of each jet stream is always found in the warm air above 

 its respective frontal surface and most often above the 500-mb. (18,000 

 feet, very nearly) position of the front. This relationship has imme- 

 diate value to the meteorologist, for, by means of it, he is able to 

 estimate the location of a high-level jet stream from temperature data 

 at the relatively low level of 500 mb., even in the absence of high-level 

 wind observations. Furthermore, knowing which front he is dealing 

 with, he can provide a reasonable estimate of the height of the axis. 

 One other feature brought out by extensive cross-section studies is 

 that the strongest winds at any level below the axis are invariably 

 found in the warmer air. 



JET STREAM WINDS 



The wind speeds in the jet-stream cross section shown in figure 2 

 are not particularly high compared with those found at lower lati- 

 tudes. Both the maritime arctic and polar jet streams consistently 

 exhibit stronger winds on any given occasion. In fact, the strongest 

 winds are found where two or more jet streams move closely to one 

 another. Although this can occur anywhere, the preferred locations 

 for such intense jet streams are the eastern coastlines of the Asian 

 and North American Continents. 



What are the highest wind speeds likely to be found in jet streams? 

 In the past, wind-speed measurements as high as 400 knots have fre- 

 quently been reported in weather messages. However, when the orig- 

 inal observations, which are obtained by balloon-tracking methods, 

 are carefully checked, they are invariably found to be in error. For 

 example, a reported 400-knot wind over Philadelphia late in January 

 1955 was checked and found to be incorrect on account of instrumental 



