INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 77 



the clifFerence of structure of its upper parts, when these are visible 

 (fibrous structure and cumuliform structure). Masses of cumulus, 

 however heavy they may be and however great their vertical devel- 



opment, should never be classed as cumulonimbus unless the whole, 

 or a part of their tops, is transformed or is in process of transfor- 

 mation into a cirrus mass. 



When a cumulonimbus covers nearly all the sky the base alone is 

 visible and resembles nimbostratus, with or without fractostratus 

 or fractocumulus below. The difference between the base of a 



