JET STREAMS — LEE 301 



altocumulus, extending from horizon to horizon, and having waves 

 at right angles to the air flow. From the ground, these clouds can 

 be observed to move at great speeds, often resulting in rapid local 

 changes in cloud cover during short intervals of time. Plate 1 shows 

 three of Dr. Schaefer's remarkable photographs of typical jet-stream 

 clouds as observed from the ground. 2 



Aloft, cloud formations at various levels can often give indications 

 of the wind direction. Under conditions of high winds, an upper 

 cloud surface will show streaks in the direction of the wind and a 

 billow structure at right angles to these streaks, in a manner analogous 

 to wind lanes on a sea surface with a superimposed transverse wave 

 pattern. 



CLEAR-AIR TURBULENCE 



It was once thought that aviation hazards, such as icing and tur- 

 bulence, were confined to the lower troposphere, and that, once aircraft 

 could fly "above the weather," all problems of flight comfort would be 

 solved. This myth exploded when high- altitude aircraft encountered 

 turbulence as violent as that encountered at low levels. The bumpi- 

 ness, or turbulence, is described by those who have experienced it to be 

 like the pounding of a fast speedboat racing across a very choppy 

 sea surface. Since there is no visual warning, it has been called 

 clear-air turbulence. 



In order to ascertain the nature of this phenomenon, many special 

 research flights have been carried out over the British Isles, Europe, 

 and the United States. Through the kind cooperation of R. C. A. F. 

 personnel, the Meteorological Service of Canada has also acquired and 

 studied numerous turbulence reports. The conclusions reached by 

 various investigators are largely in agreement, but there are also 

 contradictions which will only be resolved by further research. 



Clear-air turbulence can occur at any level of the atmosphere flown 

 thus far. It is generally found in isolated patches 50 to 100 miles in 

 length and width. These patches consist of one or more layers, the 

 vertical thicknesses of which are generally not great, being of the order 

 of 500 to 3,000 feet. On occasion, thicknesses of 6,000 feet or more 

 have been reported. Because clear-air turbulence occurs in layers, a 

 satisfactory method of moving out of turbulent air is to change alti- 

 tude by 1,500 to 2,000 feet. 



Clear-air turbulence has been found to occur in the vicinity of jet 

 streams where the wind speed varies greatly with distance in the 

 horizontal or vertical. Thus, the regions above, below, and to the left 

 of the jet axis, facing downstream, are the preferred locations of tur- 



' The writer wishes to express his gratitude to Dr. Schaefer for permission to 

 publish these photographs here. 



