clouds - colorado state university...14 clouds, or as liquid phase clouds if all portions of a cloud...

IGV99-B01 PII: S0074-6142(10)09907-9 ISBN: 978-0-12-088542-8 PAGE: 1 (1–11)

1

Chapter 1 2

Clouds 3

Clouds are pictures in the sky 4

They stir the soul, they please the eye 5

They bless the thirsty earth with rain, 6

which nurtures life from cell to brain— 7

But no! They’re demons, dark and dire, 8

hurling hail, wind, flood, and fire 9

Killing, scarring, cruel masters 10

Of destruction and disasters 11

Clouds have such diversity— 12

Now blessed, now cursed, 13

the best, the worst 14

But where would life without them be? 15

Vollie Cotton 16

1.1. INTRODUCTION 1718

Since the late 1940s, when the experiments by Langmuir (1948) and Schaefer 19

(1948) suggested that seeding of certain types of clouds could release additional 20

precipitation, there has been intensive investigation into the physics of clouds. 21

The major focus of these studies has been on the microphysical processes 22

involved in cloud formation and the production of precipitation. As the studies 23

have unraveled much about the detailed microphysics of clouds, it has become 24

increasingly apparent that these processes are affected greatly by macroscale 25

dynamics and thermodynamics of the cloud systems. We have also learned to 26

appreciate that the microphysical processes can alter the macroscale dynamic 27

and thermodynamic structure of clouds. Thus, while the focus of this book is 28

on the dynamics of clouds, we cannot neglect cloud microphysical phenomena. 29

The title of this book implies a perspective from which we view the cloud or 30

cloud system as a whole. From this perspective, cloud microphysical processes 31

can be seen as a swarm or ensemble of particles that contribute collectively, and 32

in an integrated way, to the macroscale dynamics and thermodynamics of the 33

cloud. 34

We take a similar perspective of small-scale air motions in clouds. Again, 35

we will not use our highest power magnifying lens to view the smallest scale 36

motions or turbulent eddies in clouds. We will instead examine the collective 37

1


2 CHAPTER 1 Clouds

behavior or statistical contributions of the smallest cloud eddies (i.e. those less1

than a few hundred meters or so) to the energetics of clouds and to transport2

processes in clouds.3

Following the same analogy, we view the meso-β-scale and meso-α-scale4

with a wide-angle lens, thus encompassing their contributions to the energetics5

and transport processes of a particular cloud as well as to neighboring clouds6

or cloud systems. The meso-β-scale and meso-α-scale can, for the most part,7

be considered the environment of the cloud scale that is generally the meso-γ -8

scale.9

1.2. THE CLASSIFICATION OF CLOUDS1011

Cloud types are generally defined according to the phases of water present and12

the temperature of cloud top [AMS Glossary]. Clouds are referred to as warm13

clouds, or as liquid phase clouds if all portions of a cloud have temperatures14

greater than 0 ◦C. For clouds extending above the 0 ◦C level, precipitation15

formation can be either by ice phase or droplet coalescence processes. Clouds16

consisting entirely of ice crystals are called ice-crystal clouds. Analogously,17

a cloud composed entirely of liquid water drops is called a water cloud and18

a mixed-phase cloud contains both water drops (supercooled at temperatures19

below 0 ◦C) and ice crystals, without regard to their actual spatial distributions20

(coexisting or not) within the cloud. Convective clouds extending into air21

colder than about −10 ◦C are generally mixed clouds. Supercooled droplets22

may coexist with ice particles until temperatures are cold enough to support23

homogeneous freezing or below about −40 ◦C.24

Since this text emphasizes the dynamics of clouds, it would seem25

appropriate that we adopt a classification of clouds that is based on the26

“dynamic” characteristics of clouds rather than on the physical appearance of27

clouds from the perspective of a ground observer. In fact, several scientists28

(Scorer, 1963; Howell, 1951; Scorer and Wexler, 1967) have attempted to29

design such a classification scheme based on cloud motions. However, since30

we wish to label the various cloud forms for later discussion, we shall generally31

adhere to the classifications given in the “International Cloud Atlas” (World32

Meteorological Society, 1956). This classification is based on ten main groups33

called genera, and most of the genera are subdivided into species. Each34

subdivision is based on the shape of the clouds or their internal structure.35

The species is sometimes further divided into varieties, which define special36

characteristics of the clouds related to their transparency and the arrangements37

of the macroscopic cloud elements.38

The definitions of the ten genera are as follows:39

Cirrus—Detached clouds in the form of white, delicate filaments or white or40

mostly white patches or narrow bands. These clouds have a fibrous (hairlike)41

appearance, or a silky sheen, or both.42


Chapter 1 Clouds 3

Cirrocumulus—Thin, white patches, sheets, or layers of cloud without 1

shading, composed of very small elements in the form of grains or ripples, 2

merged or separate, and more or less regularly arranged; most of the elements 3

have an apparent width of less than 1◦. 4

Cirrostratus—Transparent, whitish cloud veil of fibrous or smooth 5

appearance, totally or partially covering the sky, and generally producing 6

halo phenomena. 7

Altocumulus—White or grey, or both white and grey, patches, sheets, or 8

layers of cloud, generally with shading, composed of laminae, rounded 9

masses, or rolls, which are sometimes partially fibrous or diffuse and which 10

may or may not be merged; most of the regularly arranged small elements 11

usually have an apparent width of 1◦-5◦. 12

Altostratus—Greyish or bluish cloud sheet or layer of striated, fibrous, or 13

uniform appearance, totally or partially covering the sky, and having parts 14

thin enough to reveal the sun at least dimly, as through ground glass. 15

Altostratus does not produce halos. 16

Nimbostratus—Grey cloud layer, often dark, the appearance of which is 17

rendered diffuse by more or less continuously falling rain or snow, which 18

in most cases reaches the ground. It is thick enough to completely obscure 19

the sun. Low, ragged clouds frequently occur below the nimbostratus layer. 20

Stratocumulus—Grey or whitish, or both grey and whitish, patches, sheets, 21

or layers of cloud which almost always have dark parts, composed of 22

crenellations, rounded masses, or rolls, which are nonfibrous (except when 23

virga-inclined trails of precipitation-are present) and which may or may not 24

be merged; most of the regularly arranged small elements have an apparent 25

width of more than 5◦. 26

Stratus—Generally grey clouds with a fairly uniform base, which may 27

produce drizzle, ice prisms, or snow grains. If the sun is visible through the 28

cloud, its outline is clearly discernible. Stratus clouds do not produce halo 29

phenomena except, possibly, at very low temperatures. Sometimes stratus 30

clouds appear in the form of ragged patches. 31

Cumulus—Detached clouds, generally dense and with sharp outlines 32

developing vertically in the form of rising mounds, domes, or towers, of 33

which the bulging upper part often resembles a cauliflower. The sunlit parts 34

of these clouds are mostly brilliant white; their base is relatively dark and 35

nearly horizontal. Sometimes cumulus clouds are ragged. 36

Cumulonimbus—Heavy, dense clouds, with a considerable vertical extent, in 37

the form of a mountain or huge tower. At least part of their upper portion is 38

usually smooth, fibrous, or striated and is nearly always flattened; this part 39

often spreads out in the shape of an anvil or vast plume. Under the base of 40

these clouds, which is generally very dark, there are frequently low ragged 41

clouds and precipitation, sometimes in the form of virga. 42


4 CHAPTER 1 Clouds

In general we will not have to refer to the definitions of the clouds species or1

varieties used in the “International Cloud Atlas.” The exceptions mainly concern2

cumulus clouds, which we refer to as follows:3

Cumulus humilis—Cumulus clouds of only a slight vertical extent; they4

generally appear flattened.5

Cumulus mediocris—Cumulus clouds of moderate vertical extent, the tops6

of which show fairly small protuberances.7

Cumulus congestus—Cumulus clouds which exhibit markedly vertical8

development and are often of great vertical extent; their bulging upper part9

frequently resembles a cauliflower. We also may have occasion to refer to the10

following supplementary features and accessories of clouds:11

Mamma—Hanging protuberances, like udders, on the under surface of a12

cloud.13

Virga—Vertical or inclined trails of precipitation (fall streaks) falling from14

the base but reaching the earth’s surface.15

Pileus—An accessory cloud of small horizontal extent, in the form of a cap16

or hood above the top or attached to the upper part of a cumuliform cloud17

which often penetrates it.18

Fog is not treated as a separate cloud genus in the “International Cloud19

Atlas.” Instead it is defined in terms of its microstructure, visibility, and20

proximity to the earth’s surface as follows:21

Fog—Composed of very small water droplets (sometimes ice crystals) in22

suspension in the atmosphere; it reduces the visibility at the earth’s surface23

generally to less than 1000 m. The vertical extent of fog ranges between a24

few meters and several hundred meters.25

We include the discussion of fog in the chapter on stratocumulus clouds,26

since we shall see there is not always a clear distinction between the formative27

mechanisms of a marine stratocumulus cloud whose base is elevated from the28

surface, and a fog which reaches the surface.29

Another cloud form discussed in this text that is not treated in the30

“International Cloud Atlas” as a separate cloud genus is the orographic cloud.31

According to the “Glossary of Meteorology” (Huschke, 1959), an orographic32

cloud is a cloud whose form and extent is determined by the disturbing effects33

of orography upon the passing flow of air. Since orography can also initiate34

convective clouds, we shall often refer to a stable orographic cloud as the35

cloud form typically encountered in the wintertime during periods when the36

atmosphere is stably stratified. The cap cloud is the least complicated form of37

the orographic cloud and refers to a nearly stationary cloud that hovers over an38

isolated peak. The crest cloud is like the cap cloud with the exception that it39

hovers over a mountain ridge. The chinook arch or foehn wall cloud refers to a40

bank or wall of clouds associated with a chinook or foehn wind storm. Finally,41

the lenticular cloud, or lenticularis, is a lens-shaped cloud that forms over, or42


Chapter 1 Clouds 5

to the lee of, orographic barriers as a result of mountain waves. As the name 1

implies, lenticular clouds generally have a smooth shape with sharp outlines, 2

sometimes vertically-stacked with clear air separating each lenslike element. 3

1.3. CLOUD TIME SCALES, VERTICAL VELOCITIES, AND 4

LIQUID-WATER CONTENTS 56

In this section we examine certain cloud characteristics that have a major 7

controlling influence upon whether or not precipitation processes are important 8

and whether diabatic processes such as condensational heating and radiative 9

transfer dominate the cloud energetics. Because these physical processes affect 10

the dynamics of the cloud, it is important to recognize under what conditions 11

and in which cloud types these processes are important. 12

Saturation vapor pressure decreases when the temperature decreases (the 13

Clausius-Clapeyron relation). Cloud formation occurs when the saturation vapor 14

pressure becomes smaller than the actual partial pressure of the water vapor in 15

the air. The greater the difference, the stronger the forcing for liquid droplets or 16

ice particles to grow by vapor deposition. Clouds generally form when a buoyant 17

parcel of air is lifted (convective ascent) and cooled by adiabatic expansion. 18

As a parcel of air inside a cloud ascends, temperature decreases following a 19

moist adiabatic lapse rate which is slightly less (0.65 C per 100 m) than in 20

clear air adiabatic ascent (1 C per 100 m), because of the latent heat released by 21

condensation. The rate of condensation depends on the temperature and pressure 22

of the cloud. At 900 m and 20 ◦C, for example, it is approximately 2 g kg−123

per km of ascent. Assuming no mixing, the mixing ratio of condensed water at 24

any level above cloud base can be derived as the difference between the water 25

vapor mixing ratio at cloud base and the saturation water vapor mixing ratio 26

at that level. This is referred to as the adiabatic water-mixing ratio. Owing 27

to the mixing processes, the actual condensed water-mixing ratio is generally 28

lower than the adiabatic value. At the cloud base, the condensation of the 29

available water vapor is not instantaneous and the actual water vapor partial 30

pressure is higher than the saturation vapor pressure leading to supersaturation. 31

Supersaturation plays a critical role near cloud base for the activation of cloud 32

condensation nuclei (CCN) and ice nuclei (IN) that initiate cloud droplets or ice 33

crystals. The amount of condensed water content, either liquid or ice, is a key 34

parameter for precipitation formation. Precipitation is most likely to form in the 35

regions of largest condensed mixing ratio, i.e. in the least diluted cloud cells. 36

The macroscopic parameters of clouds that characterize precipitation and 37

diabatic process are (1) cloud time scales, (2) cloud vertical velocities, (3) 38

cloud liquid-water contents, (4) cloud temperature, and (5) cloud turbulence. 39

Time scales are important because precipitation processes are time dependent. 40

Therefore, if the cloud lifetime is too short for the time it takes to form 41

precipitation, the cloud will not precipitate even though other properties, such 42

as liquid-water content, are sufficient to support precipitation. Two time scales 43


6 CHAPTER 1 Clouds

are critical. One is the cloud lifetime, which we shall label Tc. The other, called1

the parcel lifetime, represents the time it takes a parcel to enter the cloud and2

exit its top or sides. We shall label this time scale as TP.3

Cloud vertical velocities are important because the updrafts control the time4

scale TP and determine the cloud’s ability to suspend precipitation particles. The5

magnitude of vertical velocity also provides an estimate of the wet (saturated)6

adiabatic cooling rate. For example, in the middle troposphere the wet adiabatic7

lapse rate γm is approximately 0.5 ◦C/100 m. Thus the wet adiabatic cooling8

rate CRγ is9

CRγ ' (0.5 ◦C/100 m)×W, (1.1)10

where W is the cloud vertical velocity in meters per second.11

Both the potential for precipitation formation and the cooling rates of clouds12

depend on the liquid-water content (LWC) in a cloud, for two reasons. First,13

the LWC determines the ultimate potential for a cloud to produce precipitation.14

Generally speaking, unless a cloud generates a liquid-water content in excess15

of 0.5 g m−3, it is unlikely to precipitate. Of course, other factors, such as16

aerosol concentrations (see Chapter 4) and whether the cloud is supercooled,17

also control the critical LWC for initiating precipitation. Second, the LWC is18

important because it determines the rates of shortwave or longwave radiational19

heating and cooling.20

Cloud temperature also represents an important parameter in precipitation21

potential. The cloud-base temperature indicates the liquid-water producing22

potential of the cloud. For example, a cloud with a base temperature of +20 ◦C23

has a cloud-base saturation mixing ratio of ∼15 g kg−1, while a cloud with24

a base temperature of +4 ◦C has a cloud-base saturation mixing ratio of only25

5 g kg−1. Thus, if these two clouds have equal depths, the one with the warmer26

cloud base has a much greater potential for producing rainfall. Cloud-top27

temperature is important for similar reasons, because the greater the difference28

between the cloud-base temperature and cloud-top temperature, the greater the29

potential for rainfall. Furthermore, if the cloud-top temperature is below 0 ◦C,30

then ice is possible, which greatly affects precipitation and radiation processes.31

Turbulence, the last consideration in our discussion of macroscopic cloud32

parameters, is important because it mixes properties of the cloud and interacts33

closely with the other parameters. When we speak of “characteristic” time34

scales, vertical velocities, liquid-water contents, and temperatures, the level of35

turbulence determines how representative these “characteristic” scales really36

are. For example, in some convective clouds the average updraft velocity may37

be 1 m s−1, while the standard deviation of the vertical velocity may be as large38

as 3 m s−1. The level of turbulence also affects the precipitation processes, due39

to the formation of higher peak supersaturations and to increased interactions40

among cloud particles of different types and sizes. Turbulence is also likely to41

affect the radiative properties of a cloud. In a turbulent cloud the cloud top is42


Chapter 1 Clouds 7

likely to be very lumpy, and large fluctuations in liquid water will exist. As 1

a consequence, the cloud-top radiative emittance and absorptance will differ 2

significantly from that found in a more homogeneous cloud. 3

Let us next consider how the characteristics just introduced differ in several 4

cloud forms that we will study in this book. 5

1.3.1. Fog 67

Fog may be considered the least dynamic of clouds. Fogs typically have 8

lifetimes (Tc) of 2 to 6 h. The mean vertical velocity in fog is usually quite 9

small. If we assume a mean updraft of 0.01 m s−1 for a 100-m-deep fog, 10

the time scale for a parcel entering the cloud base and exiting the cloud top 11

would be 12

TP = 100 m/0.01 m s−1= 104 s, (1.2) 13

that is, TP is on the order of 3 h. This represents the time scale in which cloud 14

microphysical processes must operate in order to generate precipitation. 15

However, the liquid-water content in fog typically ranges from 0.05 to 16

0.2 g m−3. Thus, precipitation is unlikely in all but the deepest, wettest, and 17

most maritime fogs, even though the mean vertical velocity might indicate a 18

potential for precipitation. If we use our estimate of W , of 0.01 m s−1, we 19

determine that the cooling rate due to wet adiabatic cooling is of the order of 20

CRγ = (0.5 ◦C/100 m)0.01 m s−1= 5× 10−5 ◦C s−1, (1.3) 21

which is approximately 0.2 ◦C h−1. By comparison, the rate of cooling by 22

longwave radiation flux divergence at the top of the fog can easily range from 1 23

to 4 ◦C h−1. Thus, we see that fogs can be dominated by radiative cooling. 24

The absolute magnitude of turbulence in fogs is usually small, although 25

there have been reports of vertical velocity fluctuations in some valley fogs 26

as large as 1 m s. However, if we consider turbulence in terms of fluctuations 27

from the mean motions, it appears that because both horizontal and vertical 28

mean velocities are typically small in fogs, a fog is dominated by turbulence. 29

Thus, turbulence affects transport and nearly all physical processes in fogs, even 30

though its absolute magnitude is generally small. 31

1.3.2. Stratus and Stratocumulus Clouds 3233

Stratus clouds and stratocumulus clouds do not differ markedly from fogs in 34

terms of time scales, liquid-water contents, or turbulence levels. The lifetimes 35

of stratus and stratocumulus clouds are longer, ranging from 6 to 12 h. As in 36

fog, the time scale for a parcel to enter a stratus having a mean vertical velocity 37

of 0.1 m s−1 and rising through a depth of, say, 1000 m may be 3 h. Typical 38

liquid-water contents in stratus clouds range from 0.05 to 0.25 g m−3, with 39


8 CHAPTER 1 Clouds

some maxima of over 0.6 g m−3 reported. This combination of time scales and1

liquid-water contents results in precipitation in the deepest, wettest stratus and2

stratocumulus clouds in the form of drizzle.3

Again, assuming vertical velocities of 0.1 m s−1, the wet adiabatic cooling4

rates are of the order of 2 ◦C h−1. Thus, radiation and wet adiabatic cooling5

are approximately equal contributors to the destabilization of stratus and6

stratocumulus clouds.7

The turbulence level in stratus clouds is low in absolute magnitude, just as8

it is in fog. However, since mean vertical velocities are also small, turbulence9

is a significant contributor to vertical transport processes, energetics, and the10

physics of stratus clouds.11

1.3.3. Cumulus (Humilis and Mediocris) Clouds1213

Cumulus clouds whose vertical extent may be 1500 m have a lifetime (Tc) of14

10-30 min, which is shorter than that for the preceding two types of clouds. If15

we consider an average vertical velocity of 3 m s−1, the time scale for a parcel16

to enter the cloud base and exit the cloud top is of the order of17

TP = 1500 m/3 m s−1= 500 s ' 10 min. (1.4)18

The liquid-water content of small cumuli rarely exceeds 1.0 g m−3 and is19

typically approximately 0.3 g m−3. Thus, for such short time scales and low20

liquid-water contents, precipitation is unlikely in all but the most maritime or21

cleanest airmass, wettest cumuli.22

Comparing wet adiabatic cooling rates to cloud-top radiation cooling, we23

estimate24

CRγ ' (0.5 ◦C/100 m)× 3 m s−1= 1.5× 10−2 ◦C s−1

' 50 ◦C h−1, (1.5)25

which is considerably greater than the cloud-top radiation cooling rates for26

clouds of such liquid-water contents (CRIR ∼ 4 ◦C h−1). Thus, wet adiabatic27

cooling dominates radiative effects in such clouds.28

The turbulence levels in small cumuli is relatively moderate, with root-mean-29

square (RMS) velocities ranging from 1 to 3 m s−1. Thus, turbulence plays an30

important role in such clouds.31

1.3.4. Cumulus Congestus Clouds3233

The lifetime of cumulus congestus clouds exceeds that of cumuli, from 20 to34

45 min. However, the transit time TP for a parcel entering the cloud base, rising35

at 10 m s−1, and exiting the cloud top is similar to that of small cumuli, since36

TP = 5000 m/10 m s−1= 500 s ' 10 min. (1.6)37


Chapter 1 Clouds 9

That is, higher updraft velocities in cumulus congestus clouds offset their 1

greater depth in determining TP. Because of the small TP, precipitation would be 2

unlikely if it were not for the higher liquid-water content of cumulus congestus 3

clouds, which ranges from 0.5 to 2.5 g m−3. Because the turbulence level in such 4

clouds is often quite strong, it is possible for air parcels to spend a considerably 5

longer residence time in the cloud than would be implied by TP. 6

As in the smaller cumuli, radiative effects are secondary to wet adiabatic 7

processes in the energetics of cumulus congestus clouds. 8

1.3.5. Cumulonimbus Clouds 910

Cumulonimbi are the longest living convective clouds. They have lifetimes 11

from 45 min to several hours. However, the time scale for a parcel of air to 12

enter the cloud base and commence forming precipitation before exiting the 13

top remains relatively short. Let us take, as an example, a cumulonimbus cloud 14

that is 12,000 m deep and has an average updraft velocity of 30 m s−1. The 15

Lagrangian time scale is only 16

TP = 12,000 m/30 m s−1= 400 s, (1.7) 17

which is actually less than that in the smaller cumuli. Because of the enormous 18

cooling of air parcels rising through the great depths of the cloud, typical liquid- 19

water contents in cumulonimbi range from 1.5 to 4.5 g m3 and often greater. 20

These high liquid-water contents compensate, to some extent, for the short time 21

scale. The short time scale sometimes limits the formation of precipitation, 22

which accounts for the weak echo regions (WERs) that are often observed by 23

radars. It should be noted that such intense updrafts as those present in WERs 24

are not characteristic of the entire convective storm. Because turbulence levels 25

can be so intense, there is considerable opportunity for air parcels to experience 26

much longer lifetimes than are encountered rising in the main updraft. 27

With the exception of the anvil outflow region of cumulonimbi, wet adiabatic 28

processes dominate over radiative cooling. However, radiative cooling may 29

contribute significantly to the destabilization and maintenance of the weak 30

updraft regions of cumulonimbus anvils. 31

1.3.6. Stable Orographic Clouds 3233

Let us consider now a wintertime stable orographic cloud that is above a 1400- 34

m-high mountain with a half-width of 18 km (Fig. 1.1). For this type of cloud, 35

the cloud lifetime could be many hours or even days. However, the time scale 36

for precipitation processes to operate if the winds are about 15 m s−1 is only 37

TP = 18,000 m/15 m s−1= 1200 s = 20 min. (1.8) 38


10 CHAPTER 1 Clouds

15 m s–1

18 km

1400 m

FIGURE 1.1 Schematic diagram of a stable orographic cloud.

Thus, the time scale Tp is longer than that for cumuli but considerably shorter1

than that for stratus clouds. The liquid-water contents of wintertime stable2

orographic clouds do not differ substantially from those of stratocumuli; they3

are typically less than 0.2 g m−3. It is only in highly efficient maritime clouds4

or colder and efficient ice-phase-dominated clouds that precipitate occurs.5

If we consider typical updraft speeds near the mountain barrier to be about6

1 m s−1, we have an estimate of wet adiabatic cooling rates of7

CRγ = 18 ◦C h−1, (1.9)8

which is greater by an order of magnitude than radiative cooling rates. Thus,9

near the barrier crest, wet adiabatic processes remain dominant. At distances10

removed from the barrier crest, however, where a blanket cloud may reside, or11

in weaker wind situations, one can anticipate that radiative processes become12

more significant in such clouds.13

There has been very little characterization of the levels of turbulence in14

wintertime stable orographic clouds. At cloud levels near the barrier crest,15

surface-generated turbulence could be quite significant. At higher cloud levels,16

however, turbulence levels can be expected to be relatively weaker under the17

typically stable conditions.18

We can see from these simple comparisons and contrasts that clouds form19

in a broad range of conditions that control the ultimate destiny of the cloud.20

Depending on the vertical velocity, liquid-water content, and cloud time scale,21

precipitation processes may or may not affect significantly the dynamics of22

the cloud. Similarly, radiation processes may or may not be an important23

destabilizing influence on the cloud. It should be remembered that these are only24

rough estimates and that one can expect considerable variability within a given25

cloud category. To account for such variability we must construct sophisticated26

models of each of the cloud types. In the following chapters, we present the27

foundation for constructing such models of the dynamics and physics of various28

cloud systems.29


Chapter 1 Clouds 11

REFERENCES 1

Howell, W. E. (1951). The classification of cloud forms. Clouds, fogs and 2

aircraft icing. In “Compendium of Meteorology” (T. F. Malone, Ed.), 3

pp. 1162–1166. Am. Meteorol. Soc., Boston, Massachusetts. 4

Huschke, R. E., (Ed.) (1959). In “Glossary of Meteorology” Am. Meteorol. 5

Soc., Boston, Massachusetts. 6

Langmuir, I. (1948). The growth of particles in smokes and clouds and the 7

production of snow from supercooled clouds. Proc. Am. Philos. Soc. 92, 167. 8

Schaefer, V. J. (1948). The production of clouds containing supercooled water 9

droplets or ice crystals under laboratory conditions. Bull. Am. Meteorol. Soc. 10

29, 175. 11

Scorer, R. S. (1963). Cloud nomenclature. Q. J. R. Meteorol. Soc. 89, 248–253. 12

Scorer, R. S., and Wexler, H. (1967). “Cloud Studies in Colour.” Pergamon, 13

Oxford. 14

World Meteorological Society (1956). “International Cloud Atlas, Vol. 1”, 15

Geneva. 16


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