1188 
This method, of course, has reference only to stratus 
clouds and requires nothing except a knowledge of the 
cloud thickness, which is usually available to the fore- 
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2 6 8 
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2 a 6 8 Os 234 ess 
DEWPOINT DEPRESSION (°F) 
Fia. 9.—A method of forecasting pre-warmfrontal fog or 
low stratus (after Roche [21]). The graph is entered with the 
dew-point depression and the ceiling at time of beginning pre- 
cipitation. Sloping lines give time in hours before the stratus 
forms below 600 ft, as designated by the circled numerals. 
caster. Krick has devised a method based on the use of 
(1) a recent sounding in the vicinity for which the 
information is desired, and (2) normal surface-heating 
curves during stratus conditions. The requirement of 
having both the sounding and the heating curves pre- 
vents very widespread adoption of what would other- 
wise be a reasonably precise method. 
(100 METERS ) 
GLOUD THICKNESS 
TIME TO GLEAR (HOURS) 
Fie. 10.—The relation between time required for stratus 
clouds to clear and their thickness (after Wood [26]) with data 
from several coastal stations. 
CLOUDS, FOG, AND AIRCRAFT ICING 
Still another method (Fig. 11) has been used as an 
approximation for forecasting the dissipation of fog [9]. 
This is an indirect method of taking into account the 
LOCAL TIME 
3/16 1/4 5/16 3/8 
© Wea we 
MINIMUM VISIBILITY (MILES) 
Fie. 11.—Another illustration of method for predicting 
the clearing of fog. Sloping lines are the number of hours after - 
sunrise required for clearing. (After George.) 
amount of liquid water which must be evaporated (see 
Radford’s visibility scale, Fig. 2), and the total amount 
of foggy air measured by the length of time the fog 
has been present. This method has the advantage of 
being extremely practical and dealing with easily ob- 
tained data, but possesses the disadvantage (which may 
be inherent in all methods) of requiring the clearing 
forecast to be made at a time when complete informa- 
tion about the fog is available, or may be anticipated. 
In none of these methods is any account taken of the 
nuclei characteristics; it generally is presumed that 
industrial pollution is not present to a marked degree. 
Evaluation and Recommendations. In perspective, it 
seems clear that our knowledge of the synoptic con- 
ditions under which fog forms is far from complete. 
It is almost as clear that the framework of classification 
of fog by causes is laid on a solid foundation of knowl- 
edge. This is fortunate indeed, for classifications must 
be accurate before the various causes can be isolated 
and objectively examined. Fog is so intimately con- 
nected with local geography that its forecasting must 
remain on a local basis. Much progress has been made 
in the past decade toward the establishment of local 
synoptic studies in fog, but much more remains to be 
done and refinements of existing studies are easily 
possible and highly desirable. 
In addition to continued work with local studies, 
which seems to offer the greatest immediate return for 
effort expended at present, there are several courses of 
investigation which should be pursued if any really new 
techniques in dealing with this phenomenon are to be 
developed. Of primary interest is the desirability of 
studying in great detail the vertical structure of the 
lowest layers of the atmosphere to determine the dis- 
tribution of temperature and humidity and their 
