ON THE PHYSICS OF CLOUDS AND PRECIPITATION 
By HENRY G. HOUGHTON 
Massachusetts Institute of Technology 
INTRODUCTION 
The entire science of meteorology is concerned with 
the physics of the atmosphere, but the term physical 
meteorology has been accepted as the designation for 
only one portion of the science. This division of the 
field includes topics such as atmospheric optics, at- 
mospherie electricity, solar and long-wave radiation, 
and the physical processes of condensation and pre- 
cipitation. The latter is the subject of the present con- 
tribution. The discussion will start with a consideration 
of condensation nuclei and will then proceed in turn to 
treat the initiation of condensation, the growth of the 
condensation products, and the formation of precipi- 
tation elements. A distinction must be made between 
condensation in the liquid phase and in the solid phase. 
There are also differences between the formation of 
solid and liquid precipitation elements. A brief dis- 
cussion of the artificial dissipation of fog will also be in- 
cluded. 
Some of these topics are discussed by other contri- 
butors to this volume. The purpose of the present 
contribution is to review the entire subject. For this 
reason no attempt will be made to avoid the topics 
covered by the other authors. Such duplication is not 
only essential to a complete discussion of the subject 
but may also serve to emphasize more clearly some of 
the different points of view held by various workers in 
the field. Continuity does not require a discussion of the 
artificial modification of clouds, and this topic has 
been omitted in view of the complete treatments in 
this volume by Coons and Gunn and by Schaefer. 
The subject of cloud physics has received much 
attention in recent years. This is due in large measure 
to the recent experimental work on the artificial modi- 
fication of supercooled clouds. Other stimuli have been 
the problems of aircraft icing, artificial fog dispersal, 
the propagation of microwave radio energy, and the 
detection of precipitation areas by radar. In sharp 
distinction to many other areas of meteorology, the 
problems of cloud physics are amenable to the tech- 
niques of the experimental physicist both in the labora- 
tory and in the free atmosphere. 
NUCLEI OF CONDENSATION 
The discovery of nuclei of condensation is generally 
attributed to Coulier who reported on them in 1875. 
The. pioneer in this field was John Aitken [2] whose 
work on nuclei of condensation extended from about 
1880 to 1916. C. T. R. Wilson’s name is usually coupled 
with Aitken’s, but his work has proven to be of more 
value to particle physicists than to meteorologists. 
1. In particular, reference should be made to the papers by 
Coons and Gunn, Junge, Ludlam, Moller, Nakaya, and Schaefer. 
165 
Once it was established that all natura! condensation 
requires the presence of condensation nuclei, attention 
was focused on their source, nature, and size and on their 
distribution in time and space. Many of these questions 
are still in debate. Aitken studied the subject both in 
the laboratory and in the open air. His insight mto the 
problem and his experimental techniques stand un- 
rivaled as monuments-to his name in this field. He 
developed the expansion type of nucleus counter which 
has been used in one form or another by all subsequent 
workers in the field. The careful reading of Aitken’s 
many papers [2] is an absolute requirement for any 
serious student of condensation nuclei. 
Source and Nature of Nuclei. Aitken [2] always 
referred to nuclei of condensation as ‘‘dust particles” 
although he stated clearly that they were distinct from 
the type of dust raised, for example, by high winds. 
He felt that there were two types of nuclei, those with 
an affinity for water vapor on which condensation 
begins below saturation, and nonhygroscopic nuclei 
which require an appreciable degree of supersaturation 
for the initiation of condensation. In his opinion the 
first type is the true fog-former while the second type 
usually produces only haze. Using an Aitken “dust 
counter,” Wigand [56] found that an artificial crease 
in the dust content of air, such as was produced by 
beating a carpet in a room, had no effect on the nucleus 
count. He concluded that such nonhygroscopic dust 
was inactive as condensation nuclei and proposed that 
the Aitken dust counter be renamed the kern counter, 
a suggestion that has been generally adopted. Wigand’s 
conclusions were apparently supported by kern counts 
in the presence of sand and dust storms and by Boylan’s 
laboratory studies [5]. Boylan’s results indicated that 
the introduction of dust such as coal dust and 
carbon black slightly decreased the kern count. He 
suggested that this was due to the sweeping action 
of the dust particles on the kerns. Later Junge [26] 
experimented with a wide variety of dusts, includ- 
ing some which are not wetted by water, and found 
that all dusts with a large number of particles smaller 
than about 10:4 radius increased the kern count. He 
concluded that particles of any substance can act as 
nuclei of condensation. He argued that larger particles 
would fall out in the chamber of the kern counter before 
the expansion could be made. Junge felt that earlier 
investigators did not produce dusts with a sufficient 
number of small particles to increase the kern count 
significantly. As he pointed out, a dust cloud of several 
hundred large particles in a cubic centimeter looks 
much denser than so-called ‘“dust-free” air, which may 
contain several hundred thousand ultramicroscopic con- 
densation nuclei per cubic centimeter. Junge’s results 
are very convincing and it seems reasonable to accept 
his conclusions. On the other hand, the evidence still 
