It may also happen that low dark clouds of the nature of fracto- 

 cumulus or fractostratus may grow in amount in a sky covered with 

 heavy or swelling cumulus or ^\T.th cumulonimbus and may fill the 

 spaces between the main cumulus clouds. 



Only in these two cases should (7^=9 be coded: 



A. When a cumulus cloud approaches the zenith and its upper parts 

 are stUl visible it should be coded 0^=2 or 0^=3 according as to 

 whether it is a heavy and swelling cumulus or a cumulonimbus. 



B. In damp climates it may happen that dark fractocumulus or 

 fractostratus forms a layer completely closing up the spaces between 

 the masses of heavy cumulus. In such cases one cannot distinguish 

 individual cumulonimbus clouds under their typical aspect; but their 

 passage overhead is manifest by a temporary darkening of the sky 

 and by showers. Their presence, thus made known, allows the 

 code Ci=9 to be used and not merely 0^=6. 



MIDDLE CLOUDS C^ 



0. No Middle Clouds. 



1. Typical Altostratus, Thin (15, 25). 



A sheet of this cloud resembles thick cirrostratus, from which it is 

 often derived without any break; but halo phenomena, sun pUlar, etc., 

 are not seen in altostratus, and the sun appears as though shining 

 through ground glass and does not cast shadows. 



This cloud is found in the central part of a typical disturbance. 



If there were halo phenomena or if the sun cast shadows one would 

 code Ch=5, Ch=Q, Ch = 7, or Ch=8 according to circumstances. If 

 the sun were hidden, or liable to be completely liidden by a thick part 

 of the sheet, the code Cm=2 should be used. 



2. Typical Altostratus, Thick (or nimbostratus) (26, 27). 



The sun and moon are completely hidden, at any rate by some parts 

 of the cloud sheet. Typical thick altostratus can be formed either by 

 a thickening of typical thin altostratus, (7^=1 or by the fusing to- 

 gether of the cloudlets in a sheet of altocumulus, 0^1^= 7. 



Nimbostratus is derived either by a change from typical thick 

 altostratus, or by the fusing together of the cloudlets in a sheet of 

 stratocumulus (0^=5). 



This type is found in the central parts of a typical disturbance. 



In the case of a transition from altocumulus into altostratus, if it 

 shows either in whole or part the structure of altocumulus (ripples, 

 waves, or tesselations) it is coded 0^ = 7. In the case of a change 

 from stratocumulus to nimbostratus, it will also be coded Cm=7 if the 

 lower surface shows a real relief (waves or tesselations) instead of 

 showing a smooth under surface. Thick typical altostratus and nim- 

 bostratus are often accompanied by underlying and very low clouds, 

 which are ragged and dark (fractocumulus or fractostratus); in the 

 gaps one can generally see the altostratus or the nimbostratus of a 

 lighter gray; this case will be coded as 0^ = 6, and Cm=2. If the lowest 

 clouds form a continuous sheet the observer will not assume what is 

 above, but will code 0^=6. 



3. Altocumulus, or High Stratocumulus, Sheet at One Level Only 



(28, 29). 



A cloud generally forming a single layer; it is fairly regular, and of 

 uniform thickness, the cloudlets, tesselations or waves, being alwaj^s 



25 



