THE CLASSIFICATION OF CLOUD FORMS 
the layer until latent convective instabil- 
ity is released: cast. 
b. Formed through dynamic convection. Always 
lying beneath a surface of subsidence, their 
bases parallel to it. 
(1) Fluid. Distinguished according to height: 
Sc, Ac. 
; (2) Solid: Ce. 
2. Clouds of advection. 
a. Formed by active upglide over an inversion. 
(1) Without precipitation: As. 
(2) With virga: As with vrga. 
(8) With precipitation reaching the ground: 
Ns. 
b. Formed through passive upglide over an inver- 
sion. 
(1) Without precipitation: mam. 
(2) With precipitation: Cb. (In its upper por- 
tion this becomes a cloud of thermal con- 
vection.) 
c. Formed through upglide over a free upglide , 
surface (7.e., one not resting on the ground): 
St mam. 
d. Through upglide beneath an upglide surface 
alone no clouds are formed, but rather this 
upglide favors the formation of forms under 
B, 1, 6, and otherwise leads with the help of 
thermal and dynamic convection to the forma- 
tion of St and fog beneath a surface of subsi- 
dence, if this bounds a homogeneous air mass 
that lies directly on the ground. 
3. Clouds of lifting. Whenever a maximum of rela- 
tive humidity is brought into bemg beneath an 
inversion by dynamic convection, lifting of the 
whole air mass can produce condensation there. 
a. Water. 
(1) Lifting above upthrusting Cu: pil. 
(2) Lifting through advection at a lower level: 
lent (and also, according to the preponder- 
ance of eddy diffusion before and during 
the lifting, either more stratiform or more 
cumuliform). 
b. Ice. 
(1) Lifting above upthrusting Cb: C7 pil. 
(2) Lifting of the upper layers through up- 
glide of an air mass over an upglide sur- 
face: Cs, C7. 
4. Clouds of turbulence. They arise through eddy 
diffusion in a manner similar to the forms pro- 
duced through dynamic convection. In this case, 
however, the eddy diffusion originates through 
friction with the ground. 
a. Formation made possible through evaporation 
from precipitation fallmg into the zone of 
turbulence: F's. 
b. Formation made possible through a nearly 
adiabatic lapse rate: Mc, Cu. 
C. Orographic clouds. Landforms and the conditions of 
ground friction they establish do not lead to new 
forms of cloud, but localize the regions of their 
formation. 
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1. Cloud formation because of mountains. 
a. More or less homogeneous clouds that cling to 
mountains, mostly on the windward side. 
b. Banner clouds, forming in the lee. 
2. Coastal cloudiness resulting from changed fric- 
tion. 
3. Localization of cloud formation through the dif- 
fering thermal properties of the earth’s surface. 
A genetical classification introduced by Petterssen 
[15] approaches the problem from the point of view of 
air-mass properties. His classification of four categories 
follows: 
A. Internal clouds that characterize the stability con- 
ditions of the air masses to which they belong. 
1. Clouds that form in unstable air masses: Cu hum, 
Cucon, Cb cal, Cb inc, Cb cap, Cb mam, and cer- 
tain detached high clouds such as Cz den which 
origmate from the anvils of Cb. 
2. Clouds that form in stable air masses: St and fog. 
B. Clouds that characterize the discontinuities within 
or between air masses. 
3. Clouds that form in connection with quasi-hori- 
zontal inversions. These are most frequently re- 
lated to category 2: Sc and certain types of Ac. 
A great variety of clouds belong to category 3 
but show signs of instability and are therefore 
related to category 1: Sc vesp, Sc cast, Sc cug, Cu 
und and certain types of Ac cast. 
4. Frontal clouds that are formed because of up- 
glide movement along frontal surfaces. When the 
air is stable these are of a stratiform type: As. 
When it is completely unstable they may be of 
the cumulus type: Cb arc. When it is condition- 
ally unstable, the lower part of the cloud system 
may be stratified, and Cw or Cb towers project 
from the upper part of the frontal cloud layer. 
Although not generally called such, the international 
classification of states of the sky is actually a genetical 
classification which takes as its basis three main origins 
of clouds: in cyclones, or in particular extratropical 
cyclones; in regions of convection or thunderstorm 
activity; and in regions of turbulence. The cyclonic 
cloud systems are treated in greatest detail, being 
divided into emissary, front, central, lateral, rear, and 
connecting zones. In the thundery systems, pre-thun- 
dery, thundery, and rear zones are recognized. The de- 
tails of the classification are published in standard 
references [13, 5], and need not be repeated here. It 
may be pointed out, however, that the codes for states 
of the sky, Cz, Cur, Cu, are designed specifically to 
describe the most significant aspects of nephsystems as 
they are manifest in the sky. 
A genetical classification put forward by Kahler [14] 
corresponds quite closely to the principal divisions of 
Stiive’s classification, adding but one new notion. This 
is the separate recognition of clouds formed through 
mixture of warm, moist air and cold air at an mterface 
between them. The resulting cloud forms are described 
as being weak and of the As type. 
Physical Classification. The only classification that 
has so far been advanced from the point of view of the 
