360 WAVES OF SAND AND SNOW 



explanation of the mode of formation of these 

 clouds has been given by Lord Kelvin, von 

 Helmholtz, and Sir George H. Darwin in almost 

 identical terms. The last named includes a short 

 treatment of the subject in the paper on " Ripple- 

 mark " to which reference has already been made, 

 and the subject has frequently been treated since 

 in a theoretical way. The explanation is that when 

 a " temperature inversion " occurs in the atmo- 

 sphere — i.e., when a layer of warmer air lies upon 

 a bed of colder air — then, if there be relative move- 

 ment, the surfaces become waved or rippled. Sir 

 George Darwin writes, in the paper already cited : — 

 " The layer of transition between two currents 

 of fluid is dynamically unstable, but if a series of 

 vortices be interpolated, so as to form friction- 

 rollers, as it were, it probably becomes stable. 

 ... If one of the currents of air be colder than 

 the other, a precipitation of vapour will be caused 

 in the vortices and their shapes will be rendered 

 evident by clouds. ... If one of the currents 

 veers, the axes of the vortices are shifted. The 

 existing clouds will be furrowed obliquely and the 

 vortical stratum will be cut into diamond-shaped 

 spaces determined by the intersection of the old 

 and new vortex axes. This would explain the 

 patchwork arrangement commonly observed in 

 mackerel sky." 



