METEOROLOGICAL EQUIPMENT FOR PROPAGATION STUDIES 



103 



plitier have been (iL'scrilied in Section 7.1.4. lu the 

 lightweight wiiid-aerated iiistrunieut the piece of 

 Bakclite tubing containing tlie resistors is horizontal. 

 Owing to the shape oi' 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 Section 7.1.8). The 

 latter type of balloon was also used to make ascents 

 from a boat in light and moderate winds. 



Kecently, 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 about 2 lb. 



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 3V2 lb per 100 ft. Similar cables were used for 

 a while by Eadiation Laboratory ; later on they changed 

 to the types of cable to be described presently. 



WSG 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 lb 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 lb), 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-lb 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 

 tlie winch. 



It has been found advantageous, especially for the 

 hea^■ie^ instruments, to suspend the instrument from 

 the balloon on a 100-ft fi,shline; 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 Eadiation 

 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 aboirt 6 to 8 miles per hour. A two-reel 

 technique has been developed''"' to extend the use of bal- 

 loons to somewhat higher wind speeds (from 6 to 10 

 mph) . The pull of the balloon is taken up by a separate 

 fishline, the reel of the fishline being placed windward 

 relative to the reel of the cable (Figure 3). 



In wind speeds above about 8 mph kites are used in 

 place of balloons. A small folding kite', standard for 

 "Gibson Girl" emergency radio equipment, is easy to 

 handle and requires only a light cable, but its ceiling is 

 limited to about 400 ft. This type of kite has been used 

 successfully for soundings from boats. 



A heavier, 7-ft kite^ is well adapted to land-based 

 soundings. It flies at a high angle (55° to 60°) and 

 can be put up at minimum wind speeds. At the high 

 relative wind speeds encountered in ship-based sound- 



'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. 

 'Supplied by Hoffman Radio Co., Los Angeles, Calif. 

 'Supplied by F. C. Seyfang, Atlantic City, N. J. 



