64 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



LOW CLOUDS 



(Mean upper level, 2,000 meters, 6,500 feet; mean lower level, close to the 

 surface.) 



Stratocumulus. 



Stratus. 



Nimbostratus. 



CLOUDS WITH VERTICAL DEVEI^OPMENT 



(Mean upper level, that of the cirrus; mean lower level, 500 meters, 1,600 

 feet.) 



Cumulus. 

 Cumulonimbus. 



DEFINITIONS AND DESCRIPTIONS OF THE FORMS OF CLOUDS 



Cirrus. — Detached clouds of delicate and fibrous appearance, with- 

 out shading, generally white in color, often of a silky appearance. 



Cirrus appears in the most varied forms, such as isolated tufts, 

 lines drawn across a blue sky, branching feather-like plumes (fig. 14) , 



Figure 14. Cirrus, plume-like form. 



curved lines ending in tufts (fig. 15), etc.; they are often arranged 

 in bands which cross the sky like meridian lines, and which, owing 

 to the effects of perspective, converge to a point on the horizon, or 

 to two opposite points (cirrostratus and cirrocumulus often take part 

 in the formation of these bands) . 



Cirrus clouds are always composed of ice crystals, and their trans- 

 parent character is due to the state of division of the crystals. 



As a rule, these clouds cross the sun's disc without dimming its 

 light. But when they are exceptionally thick they may veil its light 



