236 RADIO WAVE PROPAGATION EXPERIMENTS 
as low, standard, high, and very high, corresponding 
to M curves of the types substandard, standard, super- 
standard, and trapping. This analysis did not con- 
sider variations in the M curve over the path but 
rather related signal to the prevailing type of curve. On 
this basis, then, a transmission forecast for a 24-hour 
period involved the forecasting of prevailing M curves 
over the transmission path for appropriate time in- 
tervals. The length of these time intervals was deter- 
mined by the rapidity with which the weather factors 
affecting the M distribution were changing. Specific- 
ally, a 24-hour transmission forecast involved two 
M-curve forecasts plus forecasts of temperature and 
dew point trends. These forecasts were supplemented 
frequently with M-curve forecasts for times of mini- 
mum or maximum propagation conditions. These 
meteorological data could then be translated quali- 
tatively into values and trends of signal strength. This 
information was presented in the form of a graph of 
signal strength versus time. 
How the Forecast Is Made 
The forecast in general involves two determinations 
one, of the initial conditions of the air before it leaves 
land; and two, the modifications of the air by the 
water surface. A study of the synoptic situation and 
the low-level circulation reveals the location of the 
point where the air in question leaves the land. The 
synoptic situation shows the general flow pattern 
local winds from the surface to 2,000 ft indicate the 
specific pattern over the area under consideration. 
The initial temperature and moisture distributions 
are determined by studying the local hourly teletype 
sequences and radiosonde observations. The modifica- 
tion of the air is determined by considering the relation 
of the surface water temperature to the representa- 
tive air temperature and dew point, the over-water 
travel, and the rate of modification. 
Time forecasts were also made by the Army fore- 
casters. They involved straight meteorological fore- 
casts of the initial conditions to which were applied 
the space forecast technique just described. 
Ezample. This is a forecast made by the Weather 
Bureau. The synoptic weather map on the morning 
of July 26 indicated a rather weak flow of modified 
continental polar air moving in an easterly direction 
from the mainland of eastern Massachusetts out over 
the waters of Massachusetts Bay. The temperature of 
this air was potentially more than 21°C and under 
sunshine was developing surface temperatures near 
the shore line of more than 21 C by 0800. The fore- 
cast was for 1000 about 5 miles southeast of Hastern 
Point, Massachusetts. The temperature over land 
about a half-hour before this was expected to be about 
24 C, and the air flow as indicated by winds aloft was 
such as to allow the air warmed to about this figure 
to be out over this position within a half-hour. The 
lapse rate over land would be approaching the dry 
adiabatic by this time; so, as a guide, a lapse rate 
amounting to about 3C per 1,000 ft was projected 
to 1,000 ft starting from 24 C at the surface. A value 
for the sea water temperature of 17 C was predicted 
from recent observations made in the Bay. Using past 
experience, one then assumed a water modification up 
to about 300 ft, and the T curve was constructed 
starting from the surface value of 17 C, showing a 
sharp inversion at first and a gradual inversion until 
it met the guiding line representing the air from the 
land. The radio observation made at MIT about mid- 
night, July 25 to 26, was considered to be a fairly 
good check of the properties of the air mass involved. 
A surface temperature of between 21 and 22C was 
indicated. 
In forecasting the moisture curve, a value at the 
surface corresponding to the water temperature was 
made the base of the curve. Over-land dew points were 
initially predicted to be about 13.5 C, which would 
give a vapor pressure value of between 15 and 16 mb 
at the top of the water modification zone. An examina- 
tion of the raobs, both MIT and Portland, show mix- 
ing ratios of about 10.5 g per kg between 500 and 
1,000 ft, which corresponds to 15 or 16 mb. This makes 
a good check on the prevailing initial dew points. The 
raob at Albany indicated that air which was a little 
drier was moving in from the west so that a slight 
decrease in the vapor pressure was forecast between 500 
and 1,000 ft. (This part of the forecast did not prove 
to be correct, since, as the verification of the forecast 
in the figure shows, the moisture value remained 
fairly uniform from 400 up to 1,000 ft.) Another 
curve was drawn similar to the 7 curve to connect 
the surface vapor pressure value with that value at 
the top of the water modification zone, and from 
there to 1,000 ft a gradual decrease was forecast on 
the basis of the conclusions regarding the advection 
of a little dry air indicated by the midnight Albany 
sounding. 
The verification shown by the broken line in Fig- 
ure 1 turned out rather well in this instance. The 
computed and verified M curve proved to possess 
almost identical slopes throughout with the top of 
FORECAST 
——=— VERIFICATION 
JULY 26,1944 I000E WEATHER BUREAU FORECAST 27 
5 MILES SOUTHEAST OF EASTERN POINT, MASS. C77 
1000 
500 
Le 
7 19 2) 23 (4 18 20 -20 -10 0 10 20 
TEMPERATURE VAPOR PRESSURE Mo 
IN G IN MILLIBARS 
HEIGHT IN FEET 
Fiaure 1. Space forecast of M curves. 
