"MACKEREL SKY" 363 



I have often watched mackerel clouds, which 

 is best done with the aid of dark glasses. It is 

 usually the thin white clouds of the upper air 

 which are rippled, but in stormy weather dark, low- 

 lying clouds sometimes go into parallel bars trans- 

 verse to the wind. In the mackerel clouds formed 

 at great altitudes a distance of one or two degrees 

 of arc from bar to bar is a common size for the 

 smaller ripples. This, with a distance of 30,000 

 feet, means a wave-length of from 250 to 

 500 feet. In stormy weather wave-clouds of much 

 greater wave-length are often formed in which the 

 scale is so large that a few of them cross the 

 whole sky, so that they do not produce the familiar 

 " mackerel -skin " appearance. The bars of the 

 ordinary mackerel clouds are generally unsym- 

 metrical in profile, as is shown by their being denser 

 in appearance on one side than the other. This 

 shows that, like the eddies of air or water on the 

 lee of sand-waves, they have a thick and a thin 

 end. Usually they drift with the thick end up- 

 stream, but occasionally I have seen them going 

 with the thick end foremost. Sometimes they are 

 produced by the furrowing of a continuous layer 

 of cloud, but I have also seen them forming, bar 

 by bar, in clear blue sky, and on looking one 

 wave-length ahead of the last-formed member of 



