GENERAL METEOROLOGY AND FORECASTING 41 
increase with height through the fog layer. In this 
event propagation will be standard, or ducts may 
even form occasionally within the fog layer. 
COASTAL AND MARITIME CONDITIONS 
Advection is of prime importance near a coast 
where the wind may blow the air from land to sea 
or vice versa. The former case, which is the more 
important in practice, will be considered. A tempera- 
ture inversion is formed, if the air from above a 
warmer land surface flows out over a cooler ocean 
surface. Over the land the air will usually have 
attained a state of convective equilibrium with 
correspondingly slow variations of temperature and 
humidity with height. When this air comes in contact 
with the cold water surface a temperature inversion 
is formed which increases gradually as the air 
To" 32C Eo" 12.3 MILLIBAR 
Twr22C  Ey= 26.5 MILLIBAR 
HEIGHT IN FEET 
Ficure 2. Successive M curves resulting from modifica- 
tion of warm dry uir over tool moist surface. Zero time 
corresponds to the coastline; 1/4 hr, 1/2 hr, ete. refer to 
the time the air has been over water. 
proceeds over the water. Thus the inversion is the 
more pronounced, the greater the distance from the 
shore. Eventually, however, at very large distances, 
refractive index. For instance, when fog forms by 
nocturnal cooling of the ground, the total amount of 
water in the air remains substantially unchanged, 
although part of the water changes from the gaseous 
to the liquid state. It is found that water suspended 
in the air in the form of drops contributes less to the 
refractive index than the equivalent amount of vapor. 
The formation of fog, therefore, reduces the effective 
contribution of the water vapor to the refractive index. 
If there is a temperature inversion in the fog layer, 
the vapor pressure required for saturation increases 
with height, and a substandard M curve usually 
results. 
With a substandard M curve the electromagnetic 
field near the earth surface is diminished instead of 
increased, a case opposite to that of superrefraction. 
‘In practice this weakening of the field not uncom- 
monly leads to a more or less complete radio blackout. 
Fog, however, does not always produce a sub- 
‘standard M curve although that is usually the case. 
‘In certain less frequent types of fog, the temperature 
‘and saturation vapor pressure may be constant or 
equilibrium between the air and the water surface 
will again be reached. 
The temperature inversion formed during this 
process would in itself give rise to only a compara- 
tively weak duct. When, however, the air is dry, 
evaporation from the sea surface takes place simul- 
taneously with heat transfer, and a fairly strong 
negative humidity gradient is established in the 
lowest layers. This combination of temperature inver- 
sion and moisture gradient is very favorable for the 
formation of a pronounced duct off shore. 
The progressive formation of an advection duct, 
created by the mechanism just outlined, is shown 
schematically in Figure 2. The successive M curves 
correspond to a series of time intervals measured 
from the passage of the air over the shore line. The 
increase of the duct toward the maximum and the 
subsequent flattening of the M curve as the air 
approaches a new state of equilibrium is clearly seen 
from the figure. 
Duct formation in such a case depends on two 
quantities: (1) the excess of the unmodified air 
temperature above the water temperature, and (2) 
the humidity deficit, that is, the difference between 
the saturation vapor pressure corresponding to the 
water temperature and the actual water vapor pres- 
sure in the unmodified air. The problem can be 
treated by means of the mathematical theory of 
diffusion in a turbulent medium, and a considerable 
amount of effort has been spent in investigating this 
type of advective duct. Extensive mathematical 
work has been carried out in Hngland?°° based 
primarily on the large body of data on atmospheric 
diffusion gathered in connection with chemical 
warfare problems.1*? In the United States such 
ducts have been studied very extensively in con- 
nection with the propagation experiments in Massa- 
chusetts Bay where conditions are favorable for 
their formation.?°:?% 
Another phenomenon often responsible for ducts 
in coastal regions is the land and sea breeze. This 
type of wind is of thermal origin and is produced 
by temperature differences between land and sea. 
During the day, when the land gets warmer than 
the sea, the air over the land rises and that over the 
sea descends, thus causing a circulation in which 
the air in the lowest layers flows from sea to land. 
This is the sea breeze. Vice versa, during the night 
the land becomes colder than the sea, and circulation 
is in the reverse direction, creating the land breeze. 
As a rule this type of phenomenon is extremely 
shallow, and the winds do not extend above a few 
hundred feet at the most. A sea breeze modifies the 
advective conditions described above in various 
ways, and extremely strong ducts have occasionally 
been observed under sea breeze conditions. The land 
and sea breezes are of a strictly local nature and in 
some cases will extend only a few miles to land or 
sea from the shore. Nevertheless this region may be 
