METEOROLOGICAL EQIIPMENT FOR PROPAGATION STUDIES 



101 



long, and iu the other eud, dose to the pole, an aera- 

 tion fan is mounted. Wind velocity' records are made 

 b}' means of Stewart anemometers. 



Ships 



The Eoyal ]Sra\'y has detailed three yachts for atmos- 

 pheric measurements on an experimental microwave 

 transmission path over the Irish Sea. They are pro- 

 \'ided with dry and wet thermocouples at altitudes of 

 6, 10, 40, and .50 ft above sea level. The former two 

 are mounted on hinged beams outboard, while the 

 latter two are on a mast in the forward part of the 

 ship. The thermocouples are copper-constantan, and 

 there are two in series for tlie temperature measure- 

 ment with the cohl junctions phieed in a Dewar flask 

 tiUed with melting paraldehyde (nuiintaining a tem- 

 perature of .50 F). There are two pairs of dry and wet 

 junctions connected in series which measure the wet 

 bulb depression. The galvanometer is iu the ship's 

 cabin. Aeration is provided by the ship's movement, 

 and when measurements are made the ship sails into 

 the wind to minimize the effects of the discharge from 

 the smokestack. 



In the project at the Radiation Laboratory just re- 

 ferred to measurements are also being made from the 

 mast of a boat. The 48-ft mast is provided with a 6-ft 

 cross arm and a motor-aerated housing containing the 

 elements can be raised from the bottom of the mast 

 to the end of the cross arm, giving continuous infor- 

 mation over the height of its travel. 



Measurements carried out on shipboard by means 

 of captive balloons or kites will be discussed iu Sec- 

 tion 7.1.8. 



7.1.7 Measurements on Board Planes 

 and Dirigibles 



As has been mentioned before, a sling psyehrometer 

 held out of the window of a flying plane will give 

 reasonably accurate results if some elementary pre- 

 cautions are taken to insure jjroper moistening of the 

 wick. 



The two types of instruments described in Section 

 7.1.4 have been adapted for use with airplanes. In the 

 Eadiation Laboratory instruments the two elements are 

 mounted diagonally in a piece of Bakelite tubing about 

 IVz in. in diameter, the dry element in front of the 

 wet element, relative to the wind stream. In the earlier 

 airplane measurements water was blown over the moist 

 element and a reading made when the recorder showed 

 equilibrium to be reached. Now capillary action is used 



throughout, the water l)eing sujjplied fj'om a small 

 vessel underneath the Bakelite tube. This instrument 

 has been tested in a wind tunnel with wind speeds up 

 to 145 mph. The dynamic pressure effect increases 

 the reading by 0.4 C at the cruising speed of the plane 

 (100 mph). This value was checked, both in the plane 

 itself and in a wind tunnel. 



The Washington State College [WSC] instrument 

 has been adapted for airplane measurements and has 

 been used on several types of planes during tests in 

 Panama.^'' The elements were housed iu a single- 

 walled cylinder of aluminum, about 1.75 in. in diam- 

 eter, covered on the forward end with a cone. A small 

 circular opening (% in. in diameter) made by cutting 

 off' the end of the cone reduced the velocity of the air 

 across the elements to about one twenty-second of the 

 plane's speed. Comparison of a plane sounding and a 

 balloon sounding in the same region at the same alti- 

 tude and time gave identical results within reasonable 

 experimental error. 



With airplane measurements the determination of 

 the plane's altitude becomes an important task. In 

 the experimental flights at the Radiatiou Laboratory 

 the altimeter of the plane itself was used. According 

 to the experience obtained iu Panama it is desirable 

 to have an additional altimeter placed directly before 

 the operator in order to facilitate rapid and accurate 

 altitude determinations. The nominal accuracy of an 

 airplane altimeter is about 20 ft. Over sea it may be 

 possible to determine the absolute altitude of the 

 plane with about the same degree of accuracy, but 

 over land less accuracy is to be expected. 



Measurements from a dirigible (blimp) have been 

 carried out by Radiation Laboratory. The instrument 

 is suspended on a cable about 100 ft below the ship. 



^•'•^ Captive Balloon Sondes and Kites 



Radio Tkansmission Type 



Two different methods have been tried in connec- 

 tion with balloons and kites. When first used in prac- 

 tice an ordinary radiosonde was attached to the balloon 

 (kite) and the results were recorded on the ground 

 by radio in the usual way. This method was used in an 

 experimental investigation carried out under the aus- 

 pices of the AAF Board, Orlando, Fla." Although by 

 the nature of the instrument the measurements are 

 spaced 200 to 300 ft apart, a rough survey of the 

 temperature and moisture distribution sufficient for 

 some operational purposes was gained in this way. 



