AEROGRAPHER'S MATE 3 & 2 



Cloud Classification 



The international classification which has been 

 adopted by most countries is a great help to 

 pilots and meteorological personnel alike. The 

 importance of an international classification of 

 clouds cannot be overestimated, since it tends 

 to make cloud observations standard throughout 

 the world. A s for the pilot, if he can properly 

 interpret the meaning of clouds, he is usually 

 able to avoid the types which are dangerous 

 to aircraft. 



Clouds have been divided into etages, 

 genera, species, and varieties. This clas- 

 sification is based primarily on the process 

 which produced the clouds. Although clouds are 

 continually in a process of development and 

 dissipation, they nevertheless have many dis- 

 tinctive features which make this classification 

 possible. 



ETAGES. — Observations have shown that 

 clouds are generally encountered over a range 

 of altitudes varying from sea level to about 

 60,000 feet in the Tropics, to about 45,000 feet 

 in middle latitudes, and to about 25,000 feet 

 in polar regions. By convention, the part of the 

 atmosphere in which clouds are usually present 

 has been vertically divided into three etages — 

 high, middle, and low. Each etage is defined by 

 the range of levels at which clouds of certain 

 genera occur most frequently. 



Cirrus, cirrocumulus, and cirrostratus are 

 always found in the high etage. Altocumulus and 

 altostratus are found in the middle etage, but 

 altostratus may often extend into the high etage. 

 Nimbostratus is invariably found in the middle 

 etage, but may extend into the high, and es- 

 pecially the low etage. Cumulus, cumulonimbus, 

 stratus, and stratocumulus are always associated 

 with the low etage, but the tops of cumulus or 

 cumulonimbus may extend into either or both of 

 the two other etages. 



The HIGH ETAGE extends from about 10,000 

 to 25,000 feet in polar regions, 16,500 to 45,000 

 feet in temperate regions, and 20,000 to 60,000 

 feet in tropical regions. 



The MIDDLE ETAGE extends from about 

 6,500 to 13,000 feet in polar regions, 6,500 to 

 23,000 feet in temperate regions, and 6,500 to 

 25,000 feet in tropical regions. 



The LOW ETAGE extends from near the 

 earth's surface to 6,500 feet in all regions of 

 the earth. 



GENERA. — The genera of clouds are as 

 follows: 



1. Cirrus (CI): Thin featherlike clouds. 



2. Cirrocumulus (CC): Thin cotton or flake- 

 like clouds. 



3. Cirrostratus (CS): Very thin high sheet 

 cloud. 



4. Altocumulus (AC): Sheep-back- like clouds. 



5. Altostratus (AS): Medium high uniform 

 sheet cloud. 



6. Nimbostratus (NS): Dark, threatening, a- 

 morphous, and rainy layer. 



7. Stratocumulus (SC): Globular masses or 

 rolls. 



8. Stratus (ST): Low uniform sheet cloud. 



9. Cumulus (CU): Dense dome-shaped puffy 

 looking clouds. 



10. Cumulonimbus (CB): Cauliflower towering 

 clouds with cirrus veils on top. 



NOTE: Some publications show the abbrevia- 

 tions with the second letter in lower case; 

 i.e. Ci; the abbreviations used here are in 

 accordance with FMH-1. 



SPECIES. — The following are the definitions 

 of the species of clouds whose names are com- 

 monly used; others may be found in the Inter- 

 national Cloud Atlas. 



Castellanus. Clouds which present, in at 

 least some portion of their upper part, cumuli- 

 form protuberances in the form of turrets. The 

 turrets, which are generally taller than they 

 are wide, are connected to a common base. The 

 term applies mainly to cirrocumulus, altocumu- 

 lus, and stratocumulus, but especially altocumu- 

 lus. 



Stratiformis. Clouds which are spread out in 

 an extensive horizontal sheet or layer. The 

 term applies to altocumulus, stratocumulus, and 

 occasionally to cirrocumulus. 



Lenticularis. Clouds having the shape of 

 lenses or almonds, often very elongated and 

 having well-defined outlines. The term applies 

 mainly to cirrocumulus, altocumulus, and strato- 

 cumulus. 



Fractus. Clouds in the form of irregular 

 shreds, which have a clearly ragged appearance. 

 The term applies only to stratus and cumulus. 



Humilis. Cumulus clouds of only a slight 

 vertical extent; they generally appear flattened. 



Congestus. Cumulus clouds which are mark- 

 edly sprouting and are often of great vertical 

 extent. Their bulging upper part frequently re- 

 sembles cauliflower. 



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