METEOROLOGY — FORECASTING 243 
observations which were recorded, although only 17 
per cent attained the strength of class 3, 
The longest ground range observed on the S-band 
set at Tomato Hill was 200 miles on the morning of 
January 25, while the longest ground range observed 
on the SCR-271 at Crystal River was 140 miles on the 
morning of February 17. Both these ranges were the 
Imaximum permitted by the radar presentation. 
During the month of April at Tarpon Springs, 65 
per cent of 24 days’ observations showed nonstandard 
conditions, with 46 per cent giving surface return at. 
greater than 80 miles, indicating strong superrefrac- 
tion. The longest range recorded was the coast-line 
effect out to 220 miles. 
To determine suitable low-level airborne sounding 
equipment, the psychrometer equipment ML-313/AM, 
the WSC wired sonde, and sling psychrometer. ML- 
24A were mounted in aircraft L-4 (cruising speed 55 
mph) and compared with the MIT psychrograph car- 
ried by a barrage balloon. 
From considerations of the forecasting method an 
accuracy of +0.2 C in wet and dry bulb temperatures 
and a lag coefficient less than 45 sec are desirable. The 
accuracy of the MIT psychrograph is +0.2 C in tem- 
peratures, with a lag coefficient of the order of 15 sec. 
These data were gathered during hours of daylight 
and are spread rather evenly between 0900 and 1700. 
The MIT psychrograph was held at a fixed point in 
space where a conservative estimate of the fluctuations 
of temperature was 0.3 C. The airborne instruments 
integrate the measurements for a given level, hence a 
spread in the data is reasonably indicated and the 
DRY BULB TEMPERATURE 
DIFFERENCE DISTRIBUTION 
NUMBER OF POINTS _—74 
AVERAGE DIFFERENCE 0.06C 
MEDIAN 0.1 
+04 fo MODE 0.0° 
wh Co = 0.20 
TEMPERATURE IN DEGREES C 
+ 
te) 
30 
FREQUENCY OF OCCURRENCE 
40 
WET BULB TEMPERATURE 
DIFFERENCE DISTRIBUTION 
= 0.14 
50 
o —0A4 
w 
z 
3 
z 
WwW 
© +0. NUMBER OF POINTS 156 
e AVERAGE DIFFERENCE +0.08 C 
3 ‘ MEDIAN +0.1 
; ea MODE +0. 
FREQUENCY OF OCCURRENCE 
DIFFERENCE EQUALS MIT PSYCHROGRAPH LESS 
ML313 IN DEGREES CENTIGRADE 
Ficure 8. Instrument comparison. MIT psychrograph 
and psychrometer ML 313/AM on Aircraft L-4. 
statistical value of sigma may be considered represen- 
tative of the accuracy of the test instrument. 
The procedure for each test instrument involved 
making five to ten regular low-level soundings supple- 
mented by a series of passes at a fixed level. Necessary 
ground checks were carefully made using forced ven- 
tilation, and standard corrections for airborne instru- 
ments were applied. 
Psychrometer equipment ML-313/AM, consisting 
of a wet and dry bulb thermometer in a streamline 
housing, was mounted as far back in the cabin of the 
L-4 as was practical. Since the -4 is a single engine 
plane it was expected that the engine heat and propel- 
lor blast would influence the readings. The data are 
as follows: 
Dry bulb 
Number of pairs of readings: 174 
Average difference: — 0.06 C 
67% of the points agree to within 0.20 C 
Wet bulb 
Number of pairs of readings: 156 
Average difference: + 0.08 C 
67% of the points agree to within 0.14 C 
The ML-313 was, in addition, mounted on aircraft 
L-5 (single-engined, cruising speed 100 mph). The 
data are similar to those given above. The data indicate 
that, despite the expected influences of propeller blast 
and engine heat, the equipment is suitable for low- 
level soundings for propagation work. 
The WSC wired sonde was mounted on the strut 
of the L-4 some 5 ft away from the cabin of the 
plane. The comparison data follow: 
Dry bulb 
Number of pairs of readings: 140 
Average difference: + 0.10 C 
67% of the points agree to within 0.30 C 
Vapor pressure 
Number of pairs of readings: 140 
Average difference: + 0.06 mb 
67% of the points agree to within 0.98 mb 
The differences in moisture readings are rather 
larger than desirable. 
The sling psychrometer ML-24A was tested sim- 
ilarly but was considered unsuitable because of lack of 
protection from radiation. 
Of the ground-based sounding equipment used dur- 
ing the program, the MIT psychrograph consistently 
gave excellent results. The WSC wired sonde is capable 
of good results, but several mechanical difficulties 
render it unsuitable for field use by the services in its 
present form. 1t 1s expected that these difficulties will 
be ironed out in a revision of the wired sonde. 
The following statements are the personal opinions 
of the authors and do not represent the official opinion 
of the Army Air Forces Board. 
1. There is a military need for a propagation fore- 
casting service. If a radar set is to be used most effi- 
ciently, its full capabilities and limitations (including 
