Table 1 1 3. 



INTERNATIONAL CLOUD CLASSIFICATION. 



separates up in a strong wind into shreds, or if small loose clouds are visible floating 

 underneath a large Nb., the cloud may be described as Fracto-nimbns (Fr.-Nb.) 

 (" Scud " of sailors). 



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

 surface is dome-shaped and exhibits protuberances ivhile 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 Fracto- 

 cuniulus (Fr.-Cu.). 



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

 masses of cloud rising in the fonn of mountains, turrets or anvils, generally sur- 

 mounted by a sheet or screen of fibrous appearance (false Cirrus) and having at its 

 base a mass of cloud similar to nintbus. 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. 



10. Stratus (St.). — ^4 uuifomi 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 mav be distinguished by the name fracto-stratus 

 (Fr.-St.). 



During summer all low clouds tend to assume forms resembling Cumulus, and 

 may be described accordingly as Stratus cumuliformis. Nimbus cumuliformis, etc. 



The term Mammato-cumulus is applied to a cloud having a mammillated lower 

 surface, occurring especially in connection with severe local storms. 



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

 the occurrence of sirocco, mistral or foehn, is indicated by the adjective leji'ticularis, 

 e.g.. Cumulus lenticularis (Cn. lent.), Stratus Icnticularis (St. lent.). Such clouds 

 frequently show irridcscence. 



For pictures of typical cloud forms see 

 Clarke. George A. Clouds. London. 1920. 

 Great Britain, Meteorological office. Cloud forms according to the international 



classification. 2d cd. London. 1921. 

 Humphreys, William J. Fogs and clouds. Baltimore. 1926. 

 International meteorological committee. International cloud-atlas. 2d ed. Paris. 



1910. [Abridged edition for use of observers. 1930.1 

 U. S. Weather bureau. Cloud forms according to the international system of classi- 

 fication. 2d ed. Washington. 1928. 



SMITHSONIAN TABLES. 



