METEOROLOGICAL EQUIPMENT FOR SHORT WAVE 231 
perature variations with height are comparatively slow. 
The characteristic features of the atmosplieric stratifi- 
cation lie below this level. A rough survey of this 
stratification is made during the ascent. The instru- 
ment is then reeled in and is stopped at a number of 
predetermined levels, long enough to let the elements 
reach equilibrium with the surrounding air. The levels 
chosen are spaced at height intervals small enough so 
that the readings taken reveal the atmosphere struc- 
ture accurately. It has been found that rapid lowering 
of the sonde between readings will provide sufficient 
aeration of the elements to give quite accurate readings 
even in completely calm weather. 
The balloon sonde of the Navy Radio and Sound 
Laboratory uses a dry and a wet Sanborn resistor 
surrounded by a double-walled aluminum radiation 
shield. Often wind aeration is found to be sufficient for 
the wet bulb element, but in calm air the instrument 
is aerated before readings by giving the cable a series 
of rapid jerks of about 5-ft amplitude. The ground 
equipment consists of a 0 to 50 microammeter which 
can be connected to the dry element, the wet element, 
and a standard resistor in turn by means of a double- 
pole triple-throw switch. Voltage is supplied by a dry 
cell and potentiometer. 
The captive balloon sondes used by Radiation Labo- 
ratory* employ dry and wet Sanborn resistors mounted 
diagonally in a piece of Bakelite tubing surrounded 
by an aluminum radiation shield. The circuit and am- 
plifier have been described in text on p 228. In the 
lightweight wind-aerated instrument the piece of 
Bakelite tubing containing the resistors is horizontal. 
Owing to the shape of the aluminum shield it will take 
up an orientation in the wind such that the air strikes 
the dry element before the wet element. More fre- 
quently, however, they use a heavier, fan-aerated in- 
strument in which the Bakelite tubing is vertical and 
the fan is placed on top of the assembly. This instru- 
ment has been extensively used in the recent experi- 
ments at the New England coast ; either it was attached 
to a barrage balloon (35-lb lift), or in calm weather to 
a large Neoprene balloon (see text on p 230). The 
latter type of balloon was also used to make ascents 
from a boat in light and moderate winds. 
Recently, a type of captive balloon equipment has 
been developed commercially which uses the standard 
United States radiosonde recording equipment as the 
ground component. The airborne component consists 
of an audio relaxation oscillator with the measuring 
element connected in the grid circuit. Changes in the 
measured temperature or relative humidity alter the 
frequency developed by the oscillator. By means of a 
special attachment on the ground the balloon sonde is 
used in connection with the regular radiosonde receiv- 
ing and recording equipment. The airborne component 
includes dry cells for the operation of the oscillator 
and the weight of the airborne unit is approximately 
2 Ib. 
CABLE AND BALLOON TECHNIQUE 
The cable which connects the measuring elements 
aloft to meters on the ground is one of the most critical 
parts of the wired sonde. The earliest British instru- 
ment?!® used a cable obtained by stranding together 
thin, insulated, flexible copper cables; the weight is 
about 21 lb per 100 ft. Similar cables were used for 
a while by Radiation Laboratory ; later on they changed 
to the types of cable to be described presently. 
WSC developed a cable technique*® in which 
the pull of the balloon or kite is taken up by a strength 
member such as strong linen twine. Fishline, breaking 
strength 64 lb, was originally used.** Three No. 30 
enameled copper wires are wound around the strength 
member with a pitch of several inches. After being 
made up the cable was passed under thinned airplane 
dope to cement it together and make it waterproof. 
The weight of this cable is about 1 Ib. per 1,000 ft. 
Later developments in this cable resulted in three 
types that have survived accelerated tests equivalent 
to 1 year’s exposure to salt spray without developing 
serious leakage.*? 
Type A consists of a braided Fiberglas strength 
member (nominal strength 80 |b), three No. 30 For- 
mex-insulated copper wires, and a braided nylon sheath 
impregnated with vinyl plastic. 
Type B has an enameled stainless steel strength 
member (nominal strength 40 lb) and three Formex 
conductors within an impregnated nylon sheath.® 
Type C is similar to Type A but has a 180-Ib test 
Fiberglas strength member; it is used with large kites.‘ 
These cables are wound around the drum of an ordi- 
nary winch, and the conductors are connected to the 
ground equipment by means of slip rings mounted on 
the winch. 
Tt has been found advantageous, especially for the 
heavier instruments, to suspend the instrument from 
the balloon on a 100-ft fishline; this line acts as a 
buffer in protecting the instruments from sudden 
jerks of the balloon. 
Neoprene balloons" are recommended in preference 
to rubber latex balloons. They have a much longer 
useful life than rubber balloons and give warning be- 
fore breaking by becoming misshapen. The 300-g N-4 
balloon is used in connection with the WSC instru- 
ment.°? The N-700 balloon has been used by Radiation 
Laboratory for the fan-aerated instrument. Barrage 
balloons (lift 35 lb) were also successfully used in 
winds slightly in excess of those that permit the use 
of lighter balloons. 
The light type of balloon becomes unmanageable in 
winds above about 6 to 8 miles per hour. A two-reel 
technique has been developed* to extend the use of bal- 
‘Supplied by International Braid Co., Providence, R. I. 
‘Supplied by Boston Insulated Wire and Cable Co., Boston, 
Mass. 
Supplied by the Dewey and Almy Co., Cambridge, Mass. 
