JET STREAMS — LEE 299 



difficulties. The revised estimate of the maximum wind was around 

 270 knots. Recently a number of accurate wind measurements have 

 been made by aircraft flying across selected jet streams. The highest 

 reliable measurement made by this method up to November 1955 is 

 290 knots. However, it must be stressed that this figure does not 

 necessarily belie the accuracy of winds reported by other aircraft 

 not similarly equipped. A case in point is the encounter by a Comet 

 of a 350-knot wind over Tokyo. 



Another significant feature of jet streams is brought out by the ver- 

 tical cross section in figure 2 — the asymmetry of the wind distribu- 

 tion about the axis. The speeds decrease more slowly with distance 

 on the right side of the axis than on the left side, facing downstream. 

 Thus, a pilot wishing to maintain strong tailwinds would find it 

 advantageous to stay to the right of the jet-stream axis, where a 

 slight shift in location relative to it will produce little change in the 

 tailwind component. A corresponding shift on the left side of the 

 axis will result in a considerably larger decrease in the tailwind. 

 Now, on the right side of the jet stream, the wind can drop off at a 

 rate as high as 35 knots per hundred nautical miles. On the left 

 side, however, there can be a much greater rate of decrease in wind 

 speed with distance; actual measurements have shown rates as high 

 as 100 knots per hundred nautical miles. 



It is also important to know the wind-speed variations in the ver- 

 tical, or vertical wind shear. Above and below the jet axis, the wind 

 speed decreases at an average rate of 10 to 15 knots per 1,000 feet. 

 Extreme values of the vertical wind shear have been found to be as 

 high as 30 to 35 knots per 1,000 feet by B-47 flights. Generally 

 speaking, it is only necessary to fly at right angles to the wind for 

 a short distance at the same height, simultaneously taking frequent 

 observations of air temperature, to find whether one is above or below 

 the axis. If the temperature changes very little, one will know the 

 flight level is near the level of maximum wind speed. If the temper- 

 ature increases while flying to the left of the wind, one can conclude 

 that the flight level is above the level of maximum wind. Finally, if 

 the temperature decreases while flying to the left, the flight level will 

 be below the level of maximum wind. This association of the vertical 

 wind shear with the horizontal temperature field is known to meteor- 

 ologists as the "thermal-wind relationship." It has been exploited 

 by many commercial airline pilots to locate high winds on long 

 flights across the Atlantic and Pacific Oceans. By way of example, 

 Capt. Bernard C. Frost of B. O. A. C, in flying the North Atlantic 

 routes between 15,000 and 25,000 feet, found that the outside air 

 thermometer was a very valuable guide to the location of jet-stream 

 winds. Once in a strong wind at a certain altitude, he found that the 



