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notion that has persisted strongly to the present day 
in many popular works on meteorology although it 
never was given any official recognition internationally. 
The confusion resulting from a variety of irreconeil- 
able classifications led, late in the nineteenth century, 
to steps towards international agreement. A classifica- 
tion into principal cloud types, devised by Abercromby 
and Hildebrandsson, illustrated in a cloud atlas by 
Hildebrandsson, Képpen and Neumayer in 1890, was 
recommended for international use by the International 
Meteorological Congress at Munich the following year. 
It became the basis for the first International Cloud 
Atlas [10]. The ten principal cloud types recognized 
were cirrus, cirro-stratus, cirro-cumulus, alto-cumulus, 
alto-stratus, strato-cumulus, nimbus, cumulus, cumulo- 
nimbus, and stratus, in that order. 
Almost immediately after the appearance of this clas- 
sification came proposals for a more detailed one follow- 
ing the same framework. Although successive editions 
of the International Atlas appeared in 1905 and 1910 
without significant changes, official recognition of terms 
that were coming into widespread use could not be 
indefinitely postponed. In 1922 an international com- 
mission for the study of clouds was formed to rewrite 
the Atlas. 
Two new guiding principles were adopted by this 
group. The first was acceptance of the formal frame- 
work of classification, first laid down by Linnaeus, 
which had become standard in many fields of science. 
This meant the division of all forms of cloud into fami- 
lies and their further subdivision into genera, sub- 
genera, species, and varieties. The second principle was 
one suggested by Shaw, who noted that several of the 
forms were defined in terms of separate ‘‘unit clouds” 
and that several others were in terms of aggregations of 
these unit clouds. 
The work of the commission resulted in the publica- 
tion in 1929 of a revised International Cloud Atlas 
[1]. The new Atlas established four families on the basis 
of cloud height: high, middle, and low clouds, and 
clouds of vertical development. Within these four fami- 
lies the ten previously recognized forms were distrib- 
uted as genera, stratus bemg moved from tenth place 
to seventh so as to place it in the correct family. Fol- 
lowing Shaw’s suggestion, three “forms” of clouds were 
recognized: isolated clouds; sheets of clouds divided 
into filaments, scales, or rounded masses; and more or 
less continuous sheets. These forms, however, were not 
given formal standing as families or genera. A number 
of subgenera, species, and varieties were defined.on the 
basis of what was by then current usage acquired gradu- 
ally during the preceding half century without any 
great changes being made. The latest edition of the 
Atlas [13], appearing in 1932, did not change any of the 
important concepts of its predecessor. 
A different set of fundamental forms was suggested 
by Bergeron [4]: the cumuliform (Cu, Sc vesp, Cb, Ac 
cast), the wave-form (St, Sc, Ac, Cc), and the stratiform 
(Cs, As, Ns). 
1. An explanation of abbreviations is given in [13]. 
CLOUDS, FOG, AND AIRCRAFT ICING 
Genetical Classifications. The rapid advances made 
during the early years of the 20th century in the study 
of the upper air led to an increasing appreciation of the 
role of cloud forms as indicators of atmospheric proc- 
esses. This gave rise to progress along the second 
principal road in the classification of clouds, namely 
what have been called genetical classifications. Because 
there has been no international codification of genetical 
classifications, meteorologists are less likely to be famil- 
iar with all the principal ones that have been proposed, 
and this article will therefore describe them in some 
detail. 
One of the first of the genetical classifications was 
proposed by J. Bjerknes and Douglas in a memorandum 
that was later amplified by Douglas [9]. It divides 
clouds into four types on the basis of “the physical 
processes involved in the formation of clouds.”’ These 
are: 
A. Cloud systems due to slow upward motion over a 
large area, usually associated with continuous precipita- 
tion (Vs, As, Cs, and some forms of Cz). The main fea- 
ture is the great horizontal and vertical extent of the 
cloud system. 
B. The cumulus and cumulonimbus group (including 
Ci or broken As formed from anvils), due to smaller air 
masses rising through their environment. Of great in- 
terest in this group are the layers of stratocumulus or 
altocumulus which develop turreted tops and eventu- 
ally may grow into cumulonimbus. 
C. The clouds due to turbulent motion, which may 
be either irregular (fs) or arranged in definite layers 
(St, Sc, Ac, Cc). These eddy sheet clouds result from 
unstable motion within the cloudy air. Genuine stratus 
clouds are due (according to Douglas) to cooling of the 
air at the earth’s surface combined with air movement 
which sets up turbulence and raises the cloud off the 
ground. 
D. Lenticular clouds and cloud patches of smooth 
appearance, indicating local ascent of a damp stratified 
layer. 
To this classification Douglas appends the note that 
Ci clouds could not be completely classified from a 
physical point of view at that time. 
At about the same time Stiive proposed a much more 
detailed subdivision of cloud forms in a genetical classi- 
fication that he revised for a later publication in 1937 
[18]. An outline of his classification follows: 
A. Clouds formed outside the space they occupy. 
1. Fluid or solid (amorphous or crystalline): virga. 
2. Crystals: Cz, Cz une. 
B. Clouds proper to the space they occupy. 
1. Clouds of convection. Cumuliform clouds. 
a. Formed through thermal convection. 
(1) Having no connection with a surface of 
discontinuity; their bases determined by 
the condensation level of air rismg from 
the ground and therefore horizontal; the 
tops perhaps governed by an inversion: 
Cu. With strong development resulting 
from marked instability: Cob. 
(2) Formed from Sc or Ac through lifting of 
