CANTERBURY 323 



to be true in the average or climatic sense, there are many special cases, 

 such as frontal zones and land-sea breeze effects, where one would expect 

 the refractive index to change abruptly within the 80-odd kilometers of 

 horizontal distance traversed by a tangential ray passing through the 

 first kilometer in height. 



It is these latter variations that are investigated in this section. Two 

 cases of marked horizontal change of refractive index conditions were 

 studied, one which occurred over the Canterbury Plain in New Zealand, 

 and the other at Cape Kennedy, F)a. Although these particular sites 

 were chosen for several reasons, such as land-to-sea paths and subtrojMcal 

 location (where marked changes in refraction conditions are common), the 

 major consideration was that detailed aircraft and ground meteorological 

 observations were available for prolonged periods. 



These detailed measurements allow a quantitative evaluation of the 

 error apt to be incurred by assuming that the refractive index is hori- 

 zontally stratified. The procedure used was to determine the refractive 

 index structure vertically over the transmitter and assume that this same 

 structure vertically described the atmosphere everywhere. Rays were 

 then traced through this horizontally laminated atmosphere. These ray 

 paths were then compared with those obtained by the stcp-by-step ray 

 tracing through the detailed convolutions of refractive index structure in 

 the two cases under study. 



In the sections that follow we will discuss the two cases chosen for 

 study, the methods of calculation used to evaluate refraction effects, and 

 the degree of confidence to which standard prediction methods may be 

 used under conditions of horizontal inhomogeneity. 



8.2.2. Canterbury 



The Canterbury data were compiled by a radio-meteorological team 

 working from September 1946, through November 1947, on the South 

 Island of New Zealand under the leadership of R. S. Unwin [14]. This 

 report proved invaluable in this investigation, as it was very carefully 

 prepared, giving minute details of the experiment on a day-to-day basis. 

 Anson aircraft and a trawler were used for meteorological measurements 

 over the sea, and three mobile sounding trucks for observations on land. 

 The trucks and the trawler carried wired sonde equipment, whereby 

 elements for measuring temperature and humidity up to a height of from 

 150 m to 600 m (depending on wind conditions) were elevated by means 

 of balloons or kites. Standard meteorological instruments provided a 

 continuous record of wind, surface i)ressure, temi)erature, and humidity 

 at stations at the coast and 14 km and 38 km inland. The headtiuarters 

 of the [)roject were at Ashburton Aerodrome, and the observations ex- 

 tended out to sea on a fine })erpendicular to the coastline of Canterbury 



