20 TECHNICAL SURVEY 
necessary to keep track of the altitude of the plane 
by means of a carefully calibrated altimeter. 
4. Captive balloons and kites. In these devices only 
the measuring unit is carried aloft, the indicating or 
recording meters remaining at the ground. Three 
wires are required when the instrument is nonaerated 
and two additional ones when an aeration motor is 
provided. The wires are of thin insulated copper, 
stranded together into a cable, although more 
recently aluminum wires have been tried because of 
their greater mechanical strength.?35 The fine wires 
of the cable are wound in a high-pitch spiral around 
a strength member consisting of fishline and then 
glued to the latter. Considerable effort has been spent 
on the development of these cables which constitute 
the most critical part of the balloon sonde equip- 
ment 
Captive balloons are used in calm weather and in 
winds ‘not exceeding about 4 m per sec. For higher 
wind velocities the balloons become difficult to mani- 
pulate, and a kite is then used to carry the measuring 
unit aloft from the ground or even from shipboard. 
Small barrage balloons have a greater lift than 
ordinary weather balloons and can be used in the 
same winds as kites because of their streamline 
shape. They are, however, less mobile and require 
more hydrogen than the smaller balloons. 
The cable for the balloon or kite is wound on a 
drum, and connection with the stationary meters is 
made by means of slip rings. The height of the balloon 
or kite is determined by the Jength of cable paid out 
together with a rough measurement of the angle 
of the cable. 
Captive balloons reach heights of several hundred 
feet without difficulty and even heights of 1,000 to 
2,000 ft are not infrequent. 
OTHER METEOROLOGICAL 
INSTRUMENTS 
It is hardly necessary to say that measurements 
of atmospheric temperature and humidity are pos- 
sible and have been made, with instruments of a 
i 
Ea 
HEIGHT, FEET 
14 OCT 1943 
WAYLAND, MASS. 
HEIGHT, FEET 
to) 10 20 30 40 50 60 
Ficure 3. Representative standard M curve. (836 M 
units per 1,000 ft.) 
more conventional type. In the early stages of our 
knowledge of nonstandard propagation, surveys were 
made by means of an ordinary psychrometer held 
out: of the window of a slowly cruising plane and 
aerated by the slip stream. The British installations 
commonly use multijunction dry and wet thermo- 
piles which have the advantage of not requiring 
elaborate calibration. In connection with captive 
balloons this type of equipment is somewhat clumsy 
in that the cold junctions have to be carried aloft 
in a Dewar flask. 
It should be noted here that the ordinary 
noncaptive radiosonde as used in the routine meteoro- 
logical observations of the U.S. Weather Bureau and 
of the Armed Services is not suitable for radio- 
meteorological purposes. The reason is that these 
sondes are designed to give representative data only 
at definite and fairly large vertical intervals, 100 ft 
or more. These are too widely spaced to yield a 
representative M curve, as the characteristic features 
of the latter are usually concentrated in the lowest 
strata of the atmosphere. 
Wind measurements are of importance in connec- 
tion with propagation problems, for reasons which 
will be given in detail in the chapter on weather 
NEAR MARBLEHEAD 
peaEecu 
0 
Pe an = re 
() 10 20 
M-My 
"20 -10 
HEEZeoeo 
P72 
1418 
0% 8 OCT 1943 
NEAR MARBLEHEAD 
Ficure 4. Representative nonstandard M curves from the Massachusetts coast. 
