FOG 
By JOSEPH J. GEORGE 
Eastern Air Lines, Atlanta, Ga. 
Introduction 
Fog. The chapter entitled “Fog” in Byers’ General 
Meteorology |2| begins with the simple sentence, “It is 
unwise to attempt an exact definition of fog.” It is 
difficult to find a more adequate beginning. 
When a cloud consisting of minute water droplets or 
ice crystals envelops the observer and restricts his 
horizontal visibility to 1000 m or less, the international 
definition of fog has been satisfied. Under similar con- 
ditions, but with visibility greater than 1000 m the 
condition is described by the word mist, although in the 
United States this term is popularly applied to the 
condition meteorologists know as drizzle. 
Few other meteorological phenomena depend so much 
upon the location of the observer. A motorist may on 
one occasion encounter dense fog in the valleys of a 
hilly terrain and perfectly clear air as he tops the ridges. 
On another occasion when there is a very low stratus 
cloud, he may be in clear air only in the valleys and in 
fog on the ridges. Petterssen [20] points out these 
inconsistencies clearly and George [6] goes as far as to 
recommend classing together all types of low clouds 
forming under conditions which produce fog. The latter 
course has much to recommend it from the restricted 
viewpoint of aviation forecasting, but it is completely 
inadequate for many ordinary uses and impossible for 
observing practice. The conclusion seems inescapable 
that with all their clumsiness the present definitions 
are the most practical solution. 
Smoke and Haze. Any discussion of fog must include 
mention of the related phenomena of smoke and haze. 
Smoke is considered to be atmospheric pollution caused 
by the products of imperfect combustion, usually finely 
divided carbon. Haze consists of very tiny dust parti- 
cles. Against a dark background such as mountains, 
haze presents a blue appearance while mist or fog has a 
gray tone. It will be pointed out later that under many 
conditions it is impossible to tell when dry haze becomes 
moist haze, due to the action of hygroscopic nuclei, and 
when moist haze becomes fog, except for arbitrary 
visibility limits. The process is a continuous one. 
During stagnant anticyclonic conditions, when sta- 
bilizing imfluences are extremely marked, industrial 
pollution is added to the atmosphere in a normal 
amount but is confined to the lowest layers of the air 
rather than being dissipated throughout a deep layer 
and thus at times produces a high concentration of 
foreign particles in the air. The combination of fog and 
smoke under such conditions may remain day and 
night in a greater or lesser degree. This condition is 
popularly called smog. 
Physics of Fog Formation 
Since fog is a cloud which happens to form at or near 
the surface of the earth, the physics of its formation 
should not be very different from that of a true cloud. 
Accordingly it is not the purpose of this section to deal 
with cloud physics except for those features of it which 
are unique to the formation of fog, or which may differ 
from their counterparts in cloud above the ground. 
The Role of Condensation Nuclei. Through the works 
of Wilson, Aitken, Wigand, and others, it has been 
known for many years that condensation under atmos- 
pheric conditions requires the presence of some sort 
of nuclei if it is to take place at any reasonable humidity. 
At first it was concluded that ordinary dust furnished 
these nuclei, but it is now believed that only special 
types of hygroscopic nuclei are really effective. Among 
these are (1) salt particles which apparently have be- 
come airborne from sea spray and have been carried 
long distances by the wind, and (2) hygroscopic prod- 
ucts of combustion such as sulfur trioxide and possibly 
nitric oxide. According to a description by Landsberg 
[15], condensation does not take place in the absence of 
ordinary nuclei until about fourfold supersaturation is 
attained, and it is believed that this condensation takes 
place with ions as nuclei. Spontaneous condensation 
apparently begins at about eightfold supersaturation 
and takes place as droplets “‘so fine they resemble a 
cloud or fog.” 
In the presence of the hygroscopic nuclei mentioned 
above, condensation begins at humidities well below 
saturation. During the formation of a fog, the relative 
humidity is observed to increase gradually, and cor- 
respondingly the amount of suspended liquid water 
increases as more and more nuclei attract condensation 
or as already existing droplets grow larger. It is known 
that many fogs, especially those in industrial areas, 
form and exist with humidities well below 100 per cent 
and humidities as low as 80 per cent have been re- 
ported. Accordingly the process of fog formation is a 
continuous one, in which the time required for forma- 
tion is governed by the rate of increase of relative 
humidity and the quantity and type of nuclei present. 
Figure 1, adapted from Neiburger and Wurtele [16], 
illustrates this point with data gathered at the Los 
Angeles Airport under conditions in which sea salt was 
presumed to supply the nuclei. The authors point out 
that the visibility becomes approximately constant 
below a humidity of about 67.5 per cent. At this point 
they consider that the drops become crystalline and 
the obstruction to vision becomes essentially a dry 
haze. It is well known that there are nearly always 
sufficient numbers of active nuclei present to form fog 
when the humidity reaches saturation. Supersaturation 
in the production of fog is therefore generally considered 
to be limited to the order of one per cent or less when 
it exists at all. 
Willett [24] points out the fact that fogs produced at 
unusually low humidities are particularly dense and 
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