separated by clear spaces or lighter gaps ; the cloudlets are neither very- 

 large nor very dark. This layer is generally fairly stable, that is to 

 say it changes but slowly. In tropical and subtropical regions such 

 a layer often forms at the end of the night in calm weather, even at a 

 high altitude, in the absence of any disturbance. 



This layer of cloud is sometimes broken up but it is only coded as 

 (7^=3 if the layer does not proceed from the extensions of the tops of 

 the cumulus clouds; in the latter case it is coded as Cm=6. For the 

 difference between Cm=^ and Cl=5 see the explanatory remarks on 

 Ci=5. The layer of altocumulus that should be coded 0.^ = 3 is 

 distinguished from that coded (7.,/ = 5 by its stability with no tendency 

 to increase, and by a greater regularity and uniformity. 



4. Altocumulus in Small Isolated Patches, Individual Clouds Often 



Showing Signs of Evaporation and Being More or Less Lenticular 

 in Shape (30, 31). 



As regards the smallness of the cloudlets these little patches of 

 altocumulus resemble cirrocumulus, but do not show the characters 

 of clouds formed of ice crystals. Lenticular altocumulus clouds 

 show the most beautiful irisations; when this is the case they are lens 

 shaped, fairly thick, but with little or no shadows and of a pure dazzl- 

 ing white; they are slightly wavy at the edges. _ 



Generally they are scattered over the sky quite irregularly and are 

 often at different levels; they are mostly in constant change so that 

 if one looks away from the sky for only a few minutes it is difficult 

 afterward to identify the clouds previously seen. Individually they 

 are often in process of dissolution, but the amount of cloud over the 

 whole sky does not in general become greater or less. 



This type of cloud is common on the extreme lateral edge of dis- 

 turbances and also in momitainous districts under the influence of the 

 foehn wind. 



The characteristics mentioned above are very typical. Neverthe- 

 less in cases where the observer is doubtful between Cm=^ and (7^=6 

 or between 0^ = 4 and Cm = 5 they can be discriminated in the follow- 

 ing way: The sheets of altocumulus of 0^ = 4 are higher and more 

 delicate than those of C/ = 6 and they do not have the same regular 

 structure as those of Cm = 5. Wisps of rain or snow (virga) may some- 

 times be seen falling from the underside of sheets of altocumulus of 

 class C.vf = 4. 



5. Altocumulus Arranged in More or Less Parallel Bands, or an 



Ordered Layer Advancing Over the Sky (32). 



The essential feature of this type is that the sky becomes rnore and 

 more covered; the process often begins with altocumulus in large 

 parallel bands, which often have a roughly lenticular shape. On 

 their edges they may show signs of evaporating, but on the whole the 

 amount of cloud and the thickness of the sheets increases. 



Sometimes altocumulus appears from the beginning as a sheet, 

 often under the form of a vast pavement with more or less rectilinear 

 joints, and the semi transparent parts allow the blue of the sky to be 

 seen. This layer very soon thickens in places, or has another layer, 

 lower and darker, forming beneath it. 



This cloud is met with on the lateral side of a typical disturbance 

 or on the front of a weak disturbance. 



26 



