196 
mixing-ratios over ice when the subsequent growth of 
these clouds is to be examined. 
Medium- and High-Level Clouds. Medium- and 
high-level clouds are formed in air which has not re- 
cently been near the ground and in which the ice nuclei 
are likely to be especially rare. Nevertheless it appears 
that the number of active nuclei always increases as the 
temperature falls. In general, apparently stable super- 
cooled clouds are encountered at temperatures between 
OC and about —10C. At somewhat lower temperatures 
supercooled clouds commonly survive the appearance of 
ice particles, which quickly grow and fall out as Fall- 
streifen, while at temperatures below about —30C drop- 
let clouds become increasingly rare. Usually they appear 
only fleetingly before becoming rapidly transformed 
into pure ice-clouds. 
Clouds of the altocumulus type are often very shallow 
and have very small liquid-water contents, and since 
the air in the cloudlets is constantly replaced, any ice 
crystals present will grow slowly and may for some 
time produce no effect which would reveal their exist- 
ence to an observer on the ground. However, the ice 
crystals will be slower to evaporate than the droplets, 
and it is conceivable that in a shallow layer of con- 
vective circulations they may be drawn back into 
cloudlets a number of times, increasing in size and 
number all the while (because of splmter formation). 
It may be in this way that parts of altocumulus sheets 
are transformed into thin ice clouds some time after 
their formation; the ice particles frequently fall out of 
the damp layer in which the cloud had formed and 
evaporate in the drier air beneath, so that the final 
effect of the action of the ice nuclei is the dissipation 
of the cloud. This is the typical result of the introduction 
of erystal nuclei mto shallow, slightly supercooled 
clouds. On some occasions altocumulus appear to be- 
come infected with ice particles over a very limited 
area, a hole developing in the cloud layer over a patch 
of Fallstreifen [18]. This may be due to the irregular 
distribution of the ice nuclei, or may be the result of 
splinter formation following the action of a very few 
ice nuclei in a particularly large cloudlet with lower 
summit-temperatures than are reached in the other 
cloudlets. It is common for some cloudlets of a high 
altocumulus to produce Fallstreifen, and it is character- 
istically the larger cloudlets which are so affected, not 
only because of the lower summit-temperatures at which 
more ice nuclei act, but also because in the larger 
cloudlets the growth of ice particles is favoured by the 
higher water content, the greater depth of cloud, and 
the stronger upeurrent, which prevents early precipita- 
tion. 
While falling through the ice-supersaturated layer 
beneath the base of the parent cloud, Fallstrefen par- 
ticles continue their growth. In general, the depth of 
this layer and the degree of supersaturation in its upper 
part increase with fall of temperature. High, delicate 
altocumulus clouds may disappear in producing faint 
short trails of ice particles, which then continue to grow 
while descending some thousands of feet, finally becom- 
ing dense ice clouds. 
CLOUD PHYSICS 
Clearly no fundamental distinction need be made 
between ice clouds origmating im this way and cirrus 
forming from a condensation region at lower temper- 
atures with no trace at all of a parent cloud containing 
liquid water. At temperatures below about —40C the 
ice nuclei seem to be significantly more abundant; 
water droplets have a very short life and are produced 
only in clouds of convective or lenticular type in which 
updraughts of the order of a few meters per second 
occur. Nevertheless, the number of ice particles found 
in cirrus clouds by Weickmann [24] was on the average 
only about 44 em-* (droplet clouds usually contain 
several hundreds of droplets per cubic centimetre), and 
it is evident that the crystals are so few that most may 
reach a size at which they have an appreciable falling- 
speed (about 144 m sec~!). Weickmann [23] photo- 
graphed the particles of high clouds and obtained strik- 
ing evidence of the large size of cirrus particles, which 
typically consisted of radiatmg tufts of long prisms 
containing hollows and air-enclosures. This crystal form 
is an indication of rapid growth at high supersaturation, 
although the cause of the grouping into tufts is un- 
known. The crystals of cirrostratus were smaller and 
had simpler ‘‘complete” shapes. For this reason cir- 
rostratus clouds produce halos much more readily than 
cirrus, in which halo details are rather rare. 
In the initial stages of their formation cirrus clouds 
are not typically fibrous. Frequently the first details to 
appear have a patterned or granulated structure re- 
sembling that of cirrocumulus, which slowly coarsens 
and fuses while thin Fallstreifen develop, or they may 
contain waves and ripples. With steep lapse rates, 
shreds of clouds very much like fractocumulus occur; 
these sometimes develop into true cumuliform clouds 
several thousands of feet deep, but more often they 
spread out and become less dense, gradually becoming 
transformed into patches of Fallstreifen. Bright, irides- 
cent colours may be seen in such newly formed clouds 
when they are very near the sun, and there seems no 
reason to doubt that initially they contain at least some 
water droplets. On the other hand, the typical Fall- 
streifen forms of cirrus, although they often occur with 
shreds of cloud near their summits which are not truly 
fibrous and look like the vestiges of a parent cloud, do 
not always develop from such shallow initial forms. It 
is likely that these clouds often arise when especially 
suitable ice nuclei just forestall a more general con- 
densation, so that a very few crystals are able to reach 
an unusual size and falling-speed. Weickmann [24] has 
remarked on the extraordinarily small number of large 
particles (diameter up to about 400 ») which are found 
in the trails of cirrus uncinus. 
The shape of Fallstreifen trails is determined by the 
wind shear at their level and the falling-speed of their 
particles. At first the trails are usually almost vertical, 
but near their lower ends they become almost horizontal 
as the falling-speed of the evaporating particles dwindles 
and a decrease of wind causes them to lag more and 
more behind the condensation region. A simple hook- 
shape (uncinus) can thus arise with only a gradual 
wind-shear, but some cirrus trails indicate a very rapid 
