25. Typical Altostratus, Thin, M^. 



This is a typical sheet of thin altostratus, distinguished from cirro- 

 stratus by the absence of halo phenomena. The sun appears as though 

 shining through ground glass. Below this sheet of altostratus there 

 are masses of fractostratus which appear very dark. The fracto- 

 stratus clouds which form below altostratus (pi. 15), should be coded 

 ie and the altostratus ]\Ix or Mo, according to circumstances. (Photo 

 at Aberdeen, Scotland, by G. A. Clarke.) 



26. Typical Altostratus, Thick (or Nimbostratus), Ma. 



This picture shows a layer of altostratus which covers practically the 

 whole sky. Its thickness varies considerably. The sun shows in a 

 lighter patch where the fibrous structure is evident. If the sun were 

 behind the darker part of the cloud it would be completely hidden. 

 (Photo by M. Pulve in the International Atlas.) 



62 



