1186 
formation of fog under circumstances that otherwise 
would not produce fog. The air-drainage fogs which 
cause the valleys of the Allegheny chain to have one of 
the highest incidences of fog in the United States [23] 
(a) 
WIND (MPH) 
QO 4 8 12 16 20 
CEILING (100 FEET) 
(b) 
WIND (MPH) 
fo) 4 8 12 16 20 
CEILING (100 FEET) 
Fig. 5.—(a) Forecasting graph for restricted heating fog 
with rain in the trajectory (for Atlanta). Ordinates are geo- 
strophie wind velocities scaled from the noon chart prior to 
the night of fog formation. Abscissas give the minimum ceil- 
ing attained the following morning. (6) Same as (a) but no 
rain in the trajectory. 
are caused by geographic conditions. Another example 
is Charleston, W. Va., which is in a river valley opening 
to the Ohio Valley toward the northwest. Fog results 
when anticyclonic conditions prevail in such a manner 
as to prevent flow of air down the river valley and 
consequently make air drainage the paramount feature. 
In many cases, no restriction of heating is concerned; 
it may be, and frequently is, clear all day at the station 
and to windward. Methods of forecasting involve the 
use of the gradient wind direction and the moisture 
content. 
River valleys, especially low-lying marshy ground 
immediately adjacent to the river, are frequently sub- 
ject to fogs on calm nights despite a lack of cloud cover 
during the day or any special air-drainage character- 
istics. Such areas require special treatment for fore- 
casting, but so far as is known no location of this sort 
has received much study. 
Advection-Radiation Fog. Stated simply, advection- 
radiation fog is a fog which requires both factors before 
it may form. The description which follows is, with a 
few modifications, that given by George [6]. 
CLOUDS, FOG, AND AIRCRAFT ICING 
The area over which advection-radiation fogs may 
form is immediately and clearly defined as that portion 
of the coastal plams which the lower air layers may 
reach in one night’s travel from the water. A practical 
limit to gradient winds which allow fog to form is 
about 30 mph but almost never will this value exist 
both for the full distance inland and the entire time 
consumed in traveling this distance; the average trans- 
port of air will also be much less because of surface 
friction. In practice, about 200-250 miles during winter 
and 150 miles in summer were found to be the inland 
limits. This allows a maximum average transport of air 
amounting to 14-18 mph. 
AFFECTED 
SUMMER 
A 
ADDITIONAL 
AREA OTHER 
SEASONS 
Fic. 6.—The distribution of advection-radiation fog in the 
eastern United States, showing how the areas affected increase 
during the portion of the year which has longer nights. 
In Fig. 6, the double hatchmg mdicates the area 
along the Great Lakes and the eastern coast of the 
United States which is subject to advection-radiation 
fog in summer and the single hatching indicates the 
additional area affected during the remainder of the 
year. During summer, there is a strip 30-50 miles wide 
along the Gulf and Florida coasts in which no fog 
forms. The reason this exists is probably because gra- 
dient winds from sea to land, sufficient to overcome 
the tendency for land-breeze formation, allow insuffi- 
cient time for stabilization caused by radiational cooling 
to take place. If the land breeze does develop, the 
accompanying subsidence and other effects are sufficient 
to prevent the formation of fog. During the colder 
months, and even in summer farther north, the colder 
shore waters precool the air, and fog may form even 
along the coast with reasonable wind velocities. During 
winter, fall, and spring, the area which is subject to 
such fog is considerably extended, but it should be 
noted that this area has nothing to do with the fre- 
quency of formation, and, for instance, north of latitude 
37°, advection fogs are rare in winter. 
The accurate forecasting of advection-radiation fogs 
is difficult and varies markedly from place to place. 
Figure 7 illustrates how the gradient wind, no doubt 
controlled by irregular land and water surfaces, varies 
