NAMES AND HEIGHTS OF CLOUDS 



Clouds are classified according to form and appearance, but con- 

 sideration is also given to the physical processes producing them. 

 There is a general relation between the forms of clouds and their 

 heights as shown by actual measurements. The temperature, 

 moisture, and movements of the air differ characteristically at the 

 various levels above the earth's surface and hence there are significant 

 differences in the formation of clouds as well as the apparent effects 

 of distance or height above the observer. 



In the International system there are 4 families and 10 genera, 

 and in addition certain species, varieties, and special features. The 

 10 genera and their heights, as given in the International Atlas, are 

 shown in the following table, where each word preceded by a number 

 is the name of a cloud form which constitutes a distinct genus. The 

 mean heights are for temperate latitudes and refer not to sea level 

 but to the general level of land in the region. There is nearly always 

 some variation from the mean height, which serves merely as a guide 

 to the general elevation. In certain cases there may be large de- 

 partures from the mean height. 



Cirrus clouds sometimes may be observed as low as 3,000 meters 

 in temperate regions and at lower levels in higher latitudes. In the 

 polar regions, and in extremely cold weather elsewhere, ice spicules 

 are occasionally observed in the air at and near the surface. 

 When ice crystals or spicules are seen floating in the air at the surface, 

 the phenomenon is recorded as ice fog or ice crystals, depending on 

 visibility and other criteria for identifying hydrometeors. 



Clouds composed at least partially of water droplets can exist at 

 very low temperatures; sometimes low clouds are composed of a 

 mixture of water droplets and ice crystals or snow. The observer 

 must be guided by the appearance of the clouds. If they have the 

 usual appearance of cirrus forms, they should be so recorded and 

 reported in regular synoptic messages. Clouds at very low levels 

 which have the appearance of ordinary stratus clouds should be 

 recorded and reported as such, even when upper air soundings show 

 that they are made up of snow crystals, ice spicules, or a mixture of 

 either or both with water droplets. In special cases an explanatory 

 note should be entered in the record to show the composition of the 

 clouds as revealed by airplane soundings. In this connection the 

 observer is referred to the section, "Aerial views of clouds." 



CLASSIFICATION ' 

 Family A: High Clouds 



(Mean lower level, 6,000 meters, 20,000 feet) 



1. Cirrus. 



2. Cirrocumulus. 



3. Cirrostratus. 



1 Species, varieties, and special features are described in tlie sections wliich follow. 



