96 WAVES OF SAND AND SNOW 



below the freezing-point. The first observations 

 were in Hyde Park, London. On January 23 and 

 24, 1907, there were about 2 inches of snow 

 upon the ground, and snow was still falling. There 

 was a moderate breeze blowing, which in exposed 

 positions was removing snow from the surface and 

 drifting it, although not copiously. The whole sur- 

 face was covered by small ripples about i inch in 

 wave-length and ^V inch in height, which had nearly 

 vertical faces on the up-wlnd side. They there- 

 fore faced in the opposite direction to the ripples 

 formed by wind in loose, dry sand. Their motion 

 was more rapid than that of aeolian sand-ripples. 

 The movement was usually a recession, each little 

 cliff retreating owing to the removal of its substance 

 by the attack of the wind. But every now and 

 then every ripple would simultaneously advance 

 up -wind with a swift rush lasting for about one 

 second. The slower but longer-lasting retreat to 

 leeward then re-commenced. These changes 

 accompanied the gusts and lulls of the wind. 

 During the gusts there must have been an active 

 eddy with a horizontal axis and vertical whirling 

 on the weather side of each little cliff. When the 

 wind suddenly lulled it ceased to erode the cliff, 

 and the falling snowflakes, entangled in the eddy, 

 plastered up the face of the cliff, building it out 



