SNOW-WAVES AND SNOW-RIPPLES 117 



They do not have a long hfe as travelling waves, 

 for in the still nights their surface sets hard. It 

 is the setting of the surface of the snow in exposed 

 positions, which occurs without any melting and 

 re-freezing, which prevents snow -waves growing, as 

 sand-dunes grow, to large dimensions. Thus in 

 weather when the temperature never rose to the 

 melting-point, and when there was no sign of melt- 

 ing in the sun, I found that snow-waves which were 

 formed on the prairie near Winnipeg on January 

 25th had on the 28th a surface so hard that it was 

 scarcely dented by the heel of my moccasined foot. 

 I found that this resistance was due to a hard 

 surface-layer | inch thick. Pieces of this crust 

 when broken off and held up to the sunlight 

 were seen to be a mosaic of small, translucent, 

 icy blocks cemented firmly by opaque ice. In 

 copses near by the snow had no crust upon it. I 

 suggest that the setting of the drifted snow upon 

 the prairie is due to sublimation,' the lower layers 

 evaporating under the action of the earth's heat, 

 with direct condensation to the solid form in the 

 pores of the surface, which is chilled by radiation. 

 I may recall in this connection my observations 



' That is to say, the transformation of a sohd into a gas and 

 re-condensation to the solid state without the intermediate 

 formation of liquid. 



