204. RADIO WAVE PROPAGATION EXPERIMENTS 
tion exists. Because of the persistence of high-level 
anticyclonic circulation aloft, pronounced subsidence 
is maintained throughout this season; 1944, in par- 
ticular, was characterized by very low humidity above 
the 2-km level. By subsidence aloft a thermal inver- 
sion exists over a large maritime area and thus forms 
the boundary between the lower maritime polar and 
the continental tropical or superior air aloft. Varia- 
tion in height and magnitude of the inversion is the 
governing factor in daily weather’ phenomena. There 
exists a close correlation between the height of the 
base of the inversion, the pressure at 10,000 ft, and 
the lapse rate of temperature between the 5,000- and 
10,000-ft levels. It is found that with the mmtensifica- 
tion of the pressure field aloft, the lapse rate of tem- 
perature approaches the dry adiabatic condition and 
thus, under these conditions, indicates increased sub- 
sidence. Consequently tne depth of this marine struc- 
ture is diminished by the lowering of the base of the 
inversion. 
Figure 5 shows the typical structure of a moderately 
high inversion. Usually the lapse rate of temperature 
below tke base approaches, and in some cases exceeds, 
the dry adiabatic condition. This vertical mixing in- 
sures a homogeneous air mass characterized by the 
constant vapor pressure in the marine stratum. 
Figure 6 shows the typical elevated S type M curve 
for this condition. 
The discontinuity surface between the two distinct 
air masses exists over a large area. Soundings have 
been confined within a 130-mile radius of the labora- 
tory, but observations on an FC radar indicate trap- 
ping conditions existing between San Diego and Guad- 
alupe Island 225 miles to the southwest. 
4000 
SEPTEMBER 29, 1944 
3000 1502-1517 PWT 
300° TRUE 45 MILES 
2000 
FEET 
fo} 
340 360 380 400 420 440 
Ficure 6. M curve corresponding tc the inversion shown 
in Figure 5. 
Shape of the Inversion Surface 
Emphasis must be placed on the fact that, the dis- 
continuity surface is not horizontal over the area but 
is at any time a warped surface. Figure 7 shows a 
series of M curves taken by airplane to the seaward 
of the laboratory. Both the height of the inversion 
and gradients in the transitional layer vary greatly 
$000 
4000 
3000 
e 
Ww 
w 
“ 
2000 
M CURVES 
SEPTEMBER 30,1944 
260° TRUE 
1000 
340 360 380 400 420 440 460 
M 
Figure 7. M curves at different distances and times. 
480 490 
with distance. Repeated soundings indicate that the 
apparent slope is not due to large scale lowering dur- 
ing the time interval between observations. The cluster 
of M values along the mean lapse rate of M in the 
upper and lower strata indicates the homogeneity of 
the two air masses along the vector. The possibility 
of the coexistence of elevated and surface gradients 
has been considered. No significant surface discon- 
tinuities have been detected. 
There is a general tendency for the base of the in- 
version along the shore to have a maximum height at 
0800 and a minimum at 1600. Through the exchange 
of meteorological data between the University of Cali- 
. fornia at Los Angeles and the laboratory this fact is 
now fairly well established. 
Figure 8 is a plot of refractive index from a series 
of plane soundings. The diagonal lines show the 
height and distance from the base at which measure- 
ments were made. Each line is marked with the time 
of beginning and ending the flight. The numbers at 
the ends of the curves are the refractive index (n—1) 
multiplied by 10°. The indices are independent of fre- 
quency for this range. Again it is noted that conditions 
vary along the vector. 
Figure 9 is a plot of refractive index taken by air- 
plane along the San Diego-San Pedro path, indicat- 
