stratus; the cloud loses its tessellations, and takes on a fibrous struc- 

 ture; this transitional state is coded Cm = 7, as is also the transition 

 from stratocumulus to nimbostratus. It is observed in the same 

 situation as the preceding cloud. 



D. Again, altostratus, and even nimbostratus can change pro- 

 gressively into altocumulus, and in this case also the state of transition 

 is coded 0^ = 7. This phenomenon is common after continuous rain, 

 when the altostratus breaks up, that is to say at the passing away of 

 the end of the central part of a typical disturbance. It is also found 

 in the actual center when the disturbance is filling up. 



E. The opaque cloud sheets with a more or less irregular corru- 

 gated structure, too dense and tliick for the transparency of the ripples 

 to afford any criterion for their classification, should be coded as 

 C^f = 7. The wave structure, though not visible as lighter parts, is 

 nevertheless apparent, and gives a corrugated appearance to the under 

 surface. These sheets were sometimes called "wrinkled" altostratus 

 in the old classification, but tliey shoidd now be called altocumulus 

 opacus. They are met with in the same situation as classes B and 

 C above. 



These different types have this in common, that they denote a 

 weakening of a disturbance, because they are related either to one 

 which is weakened as a whole, or to a weakened, lateral part of a typical 

 disturbance. 



As regards the usually ephemeral change of altostratus, or nim- 

 bostratus to altocumulus or stratocumulus, or vice versa (class 

 C and D above) and the differences between codes Cm='2 and 0^=7, 

 see the notes on C_,i = 2. 



Corrugated altocumulus opacus (class E above) has two origins; 

 either it forms (a) like the cloud sheets of class C above, by the 

 uniting of the cloudlets and the general thickening of a layer of 

 altocumulus coded as 0^ = 5 (or more rarely 0^ = 3), or (b) as in class 

 D above, it forms from thick altostratus or from nimbostratus (Cj,/ = 2) 

 which is in process of dissolution; but it represents a definitely more 

 stable type than C or D; in case of these latter there is a quick and 

 almost direct change of the altocumulus translucidus into altostratus 

 or vice versa; in the case of class 5 the intermediate stage of alto- 

 cumulus opacus can be maintained for some time. The layer of 

 corrugated altocumulus opacus (C.,, = 7) is distinguished from sheets 

 of altocumulus coded C,„ = 3 or (7^ = 5 by its greater thickness which 

 gives it a darker color and a menacing appearance, and by the very 

 marked irregularity of the relief of the imder surface which gives it a 

 characteristic pendant appearance (festoon clouds, pendant cor- 

 rugations) . 



8. Altocumulus Castellatus, or Scattered Cumuliform Tufts (37, 38). 

 The character common to these types of altocumulus is a domed 

 shape, but they have very different aspects. 



A. Altocumulus castellatus is composed of a series of small cumuli- 

 form masses with more or less vertical development, arranged in 

 line and resting on a common horizontal base (reduced sometimes to 

 plain gray wisps); this gives the cloud a crenellated appearance. 



B. The second kind is observed in scattered tufts, white or gray, 

 but without definite shadow, with the rounded parts very slightly 

 domed; they resemble very small, more or less broken up cumulus 

 clouds without a base. 



28 



