ALTOSTRATUS OR NIMBOSTRATUS -> 

 Codes 5 & 7 



M2: Thick altostratus or ninibostratus (through 

 portions of the sheet the position of the sun or 

 moon may be indicated by a hght patch). 



The sun and moon are completely hidden 

 by at least some parts of the cloud sheet, 

 which may be fibrous in appearance. Thick 

 altostratus can be formed either by thickening 

 of thin altostratus or by the fusing together 

 of cloudlets in a sheet of altocumulus. 



Ninibostratus is derived either by a change 

 from thick altostratus or by the fusing to- 

 gether of the cloud elements in a sheet of 

 dense altocumulus, stratocumuhis, or stratus. 



When nimbostratus gives precipitation it is 

 in the form of continuous rain or snow. 

 Nimbostratus usually has a dark gray color 

 and its lower surface always has a wet ap- 

 pearance, widespread trailing precipitation, 

 "virga," which may or may not reach the 

 ground; it is quite uniform and it is not pos- 

 sible to make out definite detail. 



Code 4 



ALTOCUMULUS -» 



M3: Thin (semitransparent) altocumulus; cloud 

 elements not changing much; at a single level. 

 This cloud is fairly regular and of uniform 

 thickness. The cloudlets or waves are always 

 separated by clear spaces or lighter patches 

 and are neither very large nor very dark. 



Code 4 



ALTOCUMULUS ^ 



M4: Thin (semitransparent) altocumulus in 

 patches (often almond- or fish-shaped) ; cloud 

 elements continually changing and/or occur- 

 ring at more than one level. 



Lenticular patches often pile up in layers, 

 at times with clear spaces between. They 

 also merge horizontally in the form of rafts or 

 somewhat discontinuous sheets. 



H. O. 607 



179 



