1184 CLOUDS, FOG, AND 
lists this as an important classification of fog. One of the 
requisites for the formation of this type of fog has been 
listed by Petterssen as conditional stability of the air 
mass. If it is not stable, then vertical mixing as the air 
is lifted will add drier air from above, thus lowering the 
humidity of the surface air and destroying the chances 
of fog formation. But it is just in these stable cases that 
the air flow in the surface layers is most nearly laminar. 
In this type of flow the mixing of the different layers is 
at a minimum and consequently, in a typical case, the 
air arriving at some higher elevation upslope will be far 
more likely to consist of air which was originally well 
above the surface, consequently much drier at a point 
upwind (and downslope) and far from being in prime 
condition for fog to form. If we take the case in which 
the wind velocity causes a homogeneous turbulence 
layer to form, we still must find some method which 
will cause this layer to gain an abnormal amount of 
moisture, and strong upslope areas do not usually con- 
tain sources of moisture at the surface. 
At least in the United States, there is no source of air- 
mass properties which produces a stable, moist air 
mass of the proper qualities to form fog by being blown 
up a geographic slope. Hither the air mass is too dry, 
though stable, or it is moist and unstable. An example 
of the first type is Canadian Cp air, and an example of 
the latter is m7 air from the Gulf of Mexico. The 
region in the United States most noted for upslope fog 
is the Cheyenne area. Yet, even in 1938, Clapp [3] 
wrote, 
The importance of humidification and even saturation of 
the surface air by falling precipitation is emphasized by the 
fact that only 16% of all fogs studied occurred with a 
twenty-four hourly precipitation preceding the fog, both at 
Cheyenne and in surrounding territory, of less than .01 inch. 
In fact, the majority of fogs occur with precipitation of some 
form falling before and during the fog. 
Clapp also mentions the importance of cloud cover, and 
it seems reasonable to suspect this factor of being the 
source of much of the 16 per cent mentioned which 
occurred without precipitation. 
Despite the hallowed spot which the so-called upslope 
fogs have attained in the minds of most meteorologists, 
it seems clear that as a primary cause of fog this factor 
may be completely eliminated from consideration. This 
is not meant to minimize the importance of upslope 
cooling in the production of low visibility in pre-warm- 
frontal precipitation areas, for this is frequently an 
important factor. 
The only place at which this category should receive 
serious consideration is along the higher portions of a 
mountain range or even of an isolated mountain. Here, 
clouds form purely by upslope motion and certainly 
the resultant fogs, to the comparatively few who ex- 
perience them, must be classified as pure upslope fog. 
Postfrontal (Cold-Front) Fog. As Byers points out, this 
type of fog is rare and occurs almost entirely when the 
cold front has become nearly, or quite, stationary, and 
resembles a warm-front condition in most respects. 
There are exceptions caused by geography, as might be 
expected, and they may be important to individual 
AIRCRAFT ICING 
forecasters, but there is little profit to be gained from a 
detailed consideration of this fog classification other 
than to recognize the fact of its existence. 
Advection-Radiation Fog. One of the biggest sources of 
confusion to forecasters has always been the breadth 
with which the term “radiation fog” has been treated. 
For many years it was the custom of meteorologists 
to term a fog “radiation” if it formed at night and 
consequently required the air to lose heat by radiation. 
George [6] has poimted out the fallacy of this practice. 
Consider most locations withm a hundred or so miles 
of a temperate latitude seacoast. When conditions are 
such that air blows from the sea to the land the forma- 
tion of fog over wide areas of the coastal plains is com- 
mon and, aside from the immediate seacoast, 11 forms 
only at night. When the air flow is from land to sea, 
under clear skies, no fog forms even under perfect 
radiative conditions. Which factor is more important, 
the advection of moist air from the sea, or the loss of 
heat by radiation at night? Both of these factors are 
necessary for the formation of fog. 
It was for this very common type of fog that the 
classification advection-radiation was established. It is 
defined as fog occurring in any air mass which has been 
over an extensive water surface during the daylight 
hours preceding the night of its formation. Obviously, 
this definition includes air which was originally colder 
than the water surface as well as air which was warmer. 
In the first case, the process of fog formation involves 
the rapid addition of moisture to the air by its passage 
over the water and requires strong nocturnal cooling 
over land to stabilize the lower layers before fog can 
form. In the latter instance of warm air passing over 
cold water, evaporation is slow or nonexistent, but the 
combination of negligible heating by solar radiation 
while over the water, together with the cooling and 
stabilizing effect of the cold water surface on the lower 
layers of air, prepares these layers for fog by requiring 
little or no nocturnal cooling over land. 
It is apparent that, following a strict classification 
according to causes, two different processes are repre- 
sented here. For the purpose of scientific investigation 
it would be necessary to recognize two types, but for 
forecasting purposes they may conveniently be grouped 
together in one category and called advection-radiation 
fog. 
Radiation Fog. The question next arises as to exactly 
what types of fog should be called “radiation.” From 
the same source as the preceding discussion it seems 
reasonable to adopt the convention that pure radiation 
fog is that which forms in air that has been over land 
during the daylight hours preceding the night of its 
formation. Furthermore, there are two main conditions 
favorable for radiation fog: (1) The air has been under 
a cloud cover with or without precipitation falling 
through it during the day previous to its formation. 
(2) Pools of air cooled to an excessive degree have 
collected in valleys, causing air-drainage fogs. Although 
they are not quite the same, it appears reasonable also 
to place in this category those fogs which form in low 
marshy land and along river valleys on calm, clear 
