52 



METEOROLOGICAL MEASUREMENTS 



74 REFRACTIVE INDEX MEASUREMENTS 



The methods which have been used to make 

 refractive index measurements in the lower atmos- 

 phere are the following: 



1. Stationary installations on towers, usually with 

 automatic recording on the ground. Aerated wet and 

 dry bulb instruments are installed at several heights 

 giving a continuous survey of the M curve between 

 the ground and the top of the tower. 



2. Installations similar to (1), on shipboard, with 

 the meters or recording equipment in the ship's 

 cabin. In order to explore the humidity distribution 

 in the lowest layers adjacent to the sea surface, the 

 instruments have been mounted at the end of a beam 

 that pivots about a horizontal axis fastened to the 

 side of the ship. This device has been used extensively 

 in the Irish Sea experiments. Artificial aeration of 

 shipborne installations is not usually necessary 

 because in calm weather the necessary velocity of 

 the air is provided by the motion of the ship. 



3. Airborne installations. The unit is mounted at 

 a convenient place on the outside of the plane where 

 it is not affected by motor exhaust or propeller slip 

 stream, with the meters or recorders in the ship's 

 cabin. Comparatively slow-flying planes have been 

 used for such measurements, not only in order to 

 minimize the dynamic temperature correction, but 

 also because in a fast-flying plane too long a column 

 of air will be sampled during the period of relaxation 

 of the instrument. In airplane measurements it is 

 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. 233 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. For details the reader is referred to the reports 

 listed under Meteorological Equipment in the Bib- 

 liography (Report WPG-14). 



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 length 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. 



75 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 

 more conventional type. In the early stages of our 

 knowledge of nonstandard propagation, surveys were 



1200 



IIOO 



1000 



900 



800 



£ 700 

 u. 



jjf 600 



CD 



iu 500 



I 



400 

 300 

 200 

 100 

 O 



10 



40 



20 30 



M — Mq 



Figure 3. Representative standard M curve. (36 M 

 units per 1,000 ft.) 



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 



