Table 105. 

 INTERNATIONAL CLOUD CLASSIFICATION. 



separates up in a strong wind into shreds, or if small loose clouds ;ire visible floating under- 

 aeal b a large Nb., the cloud may be described as Fracto- nimbus (Fr.-Nb.) (" Scud " of sailors). 



8. Cumulus (Cu.), Wool pack Clouds. — Thick clouds of which the upper surface is 

 dome-shaped and exhibits protuberances while the base is horizontal. These clouds appear to be 

 formed by a diurnal ascensional movement which is almost always noticeable. When the 

 cloud is opposite the Sun, the surfaces facing the observer have a greater brilliance than the 

 margins of the protuberances. When the light falls aslant, as is usually the case, these clouds 

 throw deep shadows; when, on the contrary, the clouds are on the same side of the observer 

 as the Sun, they appear dark with bright edges. 



True cumulus has well defined upper and lower limits, but in strong winds a broken cloud 

 resembling Cumulus is often seen in which the detached portions undergo continual change. 

 This form may be distinguished by the name Fraclo-cumulus (Fr.-Cu.). 



o. Cumulo-nimbus (Cu.-Nb.), The Thunder-Cloud; Shower-Cloud. — Heavy masses 

 of cloud rising in the form of mountains, turrets or anvils, generally surmounted by a sheet or screen 

 of fibrous appearance (false Cirrus) and having at its base a mass of cloud similar to nimbus. 

 From the base local showers of rain or snow (occasionally of hail or soft hail) usually fall. 

 Sometimes the upper edges assume the compact form of cumulus, and form massive peaks 

 round which delicate " false Cirrus " floats. At other times the edges themselves separate 

 into a fringe of filaments similar to Cirrus clouds. This last form is particularly common in 

 spring showers. 



The front of thunder-clouds of wide extent frequently presents the form of a large arc 

 spread over a portion of a uniformly brighter sky. 



io. Stratus (St.). — A uniform layer of cloud resembling a fog but not resting on the ground. 

 When this sheet is broken up into irregular shreds in a wind, or by the summits of mountains, 

 it may be distinguished by the name Fracto-stralus (Fr.-St.). 



I )uring summer all low clouds tend to assume forms resembling Cumulus, and may be de- 

 scribed accordingly as Stratus curmUiformis, Nimbus cumuliformis, etc. 



The term Mammato-cumulus is applied to a cloud having a mammillatcd lower surface, 

 occurring especially in connection with severe local storms. 



The ovoid form, with sharp edges, assumed by certain clouds, particularly during the oc- 

 currence of sirocco, mistral or foehn, is indicated by the adjective lenticular is, e.g., Cumulus 

 lenticularis (Cu. lent.), Stratus lenticularis (St. lent.). Such clouds frequently show iridescence. 



For pictures of typical cloud forms see "International Cloud Atlas," 2(1 ed., Paris, 10 to; 

 also U.S. Weather Bureau, "Classification of Clouds for the Guidance of Observers," Wash- 

 ington, D.C., 1911, and Gt. Britain, Meteorological Office, "Observer's Handbook," London 

 (annual). 



Smithsonian Tables. 



235 



