240 RADIO WAVE PROPAGATION EXPERIMENTS 
impracticable to forecast accurately four differenc 
classes of propagation, but forecasting on a basis of 
two classes, standard and nonstandard, can and 
should be done. 
During the first part of the over-land program an 
attempt was made to forecast the specific M curves 
as shown by the Leesburg and New Port Richey 
sounding stations and to correlate these curves with 
the two to four propagation classes outlined for each 
radar unit. However, because of the wide variation 
of surface terrain (sand, swamps, lakes, forests) in 
this general area, no single sounding was necessarily 
representative of the entire air mass, since subsid- 
ence and radiation effects almost certainly varied con- 
siderably over the different kinds of terrain surround- 
ing the sounding locations. On this basis, then, rather 
than attempting to forecast a hypothetical representa- 
tive M curve, a series of correlations was made relat- 
ing the general synoptic situation directly to the radar 
performance. Using this method, the actual over- 
land forecasting results showed that, while approxi- 
mately 80 to 85 per cent of the total operating hours 
could be correctly forecast as either standard or non- 
standard propagation periods, only some 50 per cent 
of the nonstandard hours could be forecast correctly. 
This is only a little better than climatology, and 
more work remains to be done on the over-land fore- 
casts of propagation variations. 
In addition ‘to the ground clutter verification of 
superrefraction, several low-level coverage flights were 
made from Leesburg to Crystal River and some 80 
miles out into the Gulf at an altitude of 100 ft, re- 
turning at 1,000 ft to check coverage above duct levels. 
Only a very few flights were made during periods of 
extended propagation, but during these periods, while 
interference from extended ground clutter prevented 
detection of the plane over land, extended ranges were 
recorded for the VHF (very high frequency) air-to- 
ground communication contact. 
In a further attempt to investigate some of the 
operational possibilities of trapping conditions at low 
and intermediate altitudes, measurements were taken 
of maximum ranges on the airborne X-band APQ/13 
radar during routine flights. However, as the ranges 
observed were very erratic, no conclusions could be 
drawn. In this respect it should be stated that while 
excellent cooperation was obtained in getting various 
radar and aircraft observations, the project had a 
low priority and as a consequence could not fully in- 
vestigate many of the more important operational 
possibilities which would have required extensive use 
of radar and aircraft facilities. 
The over-water forecasting program at Tarpon 
Springs was set up to compare the results of fore- 
casts made under field conditions of limited meteoro- 
logical data with those made using all available mete- 
orological information given in the forecasting sys- 
tem presented in reference 3. This system was based 
primarily on the over-water modification studies 
presented at the last conference, where duct height d 
was related to the wind speed at 1,000 ft, the distance 
of over-water travel, and the I deficit, which is the 
difference between the M value at some reference level, 
in this case the sea surface, and the M value of the 
unmodified air reduced to sea level. In general, no 
attempt was made to forecast the specific lapse rates 
and M curves corresponding to the current meteoro- 
logical situation. With uniform weather conditions 
existing over water, there was assumed a 100 per cent 
correlation betwen the M curve and the corresponding 
radar performance, so that it was merely necessary 
to determine the representative d and AM of the air 
mass to obtain the complete propagation forecast. 
As it was impracticable to establish a permanent 
target in the Gulf of Mexico beyond the radar hori- 
zon, the existence of superrefraction was generally 
assumed when there was extended appearance of 
coast-line clutter. It is realized that such an effect 
may not be representative of open water conditions 
because of the possibility of local sea breezes giving 
rise to the necessary temperature and moisture gradi- 
ents for trapping. However, on this basis, 18 out of 
20 forecasts correctly verified the presence or absence 
of extended coast-line clutter to within 1 to 2 hours 
of the total duration. With extended echoes existing 
during 55 per cent of the test days over the coastline, 
this accuracy is considerably greater than could be 
arrived at by any purely statistical procedure. It 
should be stated here that these forecasts proved to 
be particularly valuable to the radar personnel, since 
certain. engineering tests in progress on the radars 
made an accurate evaluation of the effects of super- 
refraction on the radar set performance necessary 
during the calibration flights. 
As an additional check on the existence of super- 
refraction over water, forecasts were made of the 
ranges for S-band radars and VHF communication 
on low-level coverage flights into the Gulf. Of a total 
of ten flights, six were made during periods of ex- 
tended coast-line return, three of which were cor- 
rectly forecast as giving superrefraction on S-band 
radar and two as giving increased ranges on VHF 
communication. Although this is not so accurate as 
the forecast of surface effects, a large error may have 
been introduced by the fact that the forecasted duct 
heights were of the same order of magnitude as the 
lowest levels attained by the plane in its flight over 
the Gulf. All the over-water flights showed normal 
horizon ranges at 1,000- to 3,000-ft levels on the re- 
turn legs. 
In another attempt to determine the vertical cov- 
erage patterns resulting from low-level nonstandard 
propagation, several free balloon flights were made. 
Standard weather service reflectors were attached to 
