180 WAVES OF SAND AND SNOW 



wind -current leaves the surface at the crest and, 

 descending, strikes it again somewhere on the 

 weather slope of the next ridge. The surface 

 current of the eddy sweeps backwards from that 

 point to the preceding crest. The steep lee slope 

 and the gentle weather slope of the sand surface 

 show the blunt head and the fine tail of the lower 

 half of the eddy. The measurements of sand-waves 

 and sand-ripples showed their wave-length to be 

 eighteen times as great as their height. If the 

 node, or point of divergence of the surface currents, 

 which is at the tail end of the eddy, be halfway 

 between the trough and crest, then the length of 

 the eddy is nine times as great as the height of the 

 sand -wave and, as the height of! the eddy cannot 

 be much greater than the height of the sand -ridge, 

 the eddy must be about nine times as long as it 

 is high. The core of air which revolves under the 

 lee of each broad sand ridge has therefore a cigar 

 or torpedo -shaped profile, with its blunt end up- 

 wind . 



A group of snow-waves near Winnipeg, with 

 loose snow below the troughs, had a wave-length 

 fifty times as great as the height. The eddy-space 

 had a blunt head and fine tail form, and the length 

 of the eddy must have been about twenty -five times 

 as great as the height. 



