INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 67 



At the lower levels, where altocumulus may be derived from a 

 spreading out of the tops of cumulus clouds, it may easily be mistaken 

 for stratocumulus ; the convention is that the cloud is altocumulus if 

 the smallest well-defined and regularly arranged elements which 



are observed in the layer (leaving out the detached elements which 

 are generally seen on the edges) are not greater than 10 solar di- 

 ameters in their smallest diameters. 



When the edge of a thin semitransparent patch of altocumulus 

 passes in front of the sun or moon a corona appears close up to them ; 



