302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



bulence. The air in the core of the jet stream and to its right is smooth 

 by comparison. If an aircraft is flying parallel to a jet stream, an 

 attempt should be made to fly on the right side of the jet axis, because 

 not only would there be a smaller chance of encountering turbulence, 

 but also there would be the added advantage of maintaining strong 

 tail winds. 



The frequency of various intensities of turbulence has been studied 

 by J. Clodman, of the Meteorological Division. Analysis of more 

 than 500 reports of aircraft turbulence over a height range of 18,000 

 to 45,000 feet revealed the following results. For three stations where 

 reports of nonoccurrences were also made, about a quarter of all flights 

 encountered turbulence. Fifty -two percent of these occurrences were 

 classed as light, 25 percent as moderate, 5 percent as heavy, and 3 per- 

 cent as severe. The remainder were classified as light to moderate or 

 moderate to heavy. Hence the majority of these occurrences were in 

 the light or moderate range. The few cases of moderate and heavy 

 turbulence occurred in laj'ers not greater than 3,500 feet in depth, in 

 agreement with the results obtained in Britain. 



A comparison of the frequency of turbulence reports at each level 

 with the frequency of time flown at each level showed that they were 

 almost identical, from which it is inferred that the probability of en- 

 countering turbulence at any level from 18,000 to 45,000 feet is 

 approximately the same. 



A study of turbulence reports collected on Canberra test flights over 

 Britain was described by Eric Hyde, test pilot of Short Bros, and 

 Harland Ltd., of Belfast, in the April 1954 issue of "Flight." The 

 general conclusions are similar to those reached elsewhere. However, 

 they do report that the intensity of turbulence decreased with increas- 

 ing height. For example, all cases of severe and violent turbulence 

 were encountered below 30,000 feet, the area most affected being around 

 25,000 to 29,000 feet. The highest recorded altitude of turbulence was 

 49,000 feet, where only light turbulence was felt. Only rarely was 

 turbulence encountered above the tropopause, and it was never greater 

 than moderate. In contrast to experience elsewhere, there were many 

 flights through well-documented jet streams which yielded no trace 

 of turbulence at all. 



