DISCUSSION OF BASIC DATA 3 



standard height levels was then computed as the 

 arithmetic mean of the individually interpolated 

 values of N at each standard height for all of 

 the profiles in the sample. In this way, 5-year 

 mean values of refractivity were obtained for 

 a large number of levels, ranging from the sur- 

 face to 30 km (or more) above the surface, for 

 a worldwide sample of stations. Over 18,000 in- 

 dividual values of mean N were determined in 

 this way. 



In addition to these calculations, the approxi- 

 mate height of the tropopause was determined 

 for each profile ; this was the height of the bot- 

 tom of the lowest atmospheric layer which met 

 the following criteria: 



(a) the layer thickness was ^ 2 km, 



(b) the temperature gradient ^ -2°C/km. 

 The 2-km thickness could be made up of two or 

 more consecutive layers from the radiosonde 

 data. Mean monthly values of tropopause 

 heights were obtained for the period of record 

 for each station by averaging the individual 

 profile tropopause heights ; maps of these tropo- 

 pause heights are shown in appendix D. 



The monthly mean value of the average N- 

 gradient between the surface and 1 km above 

 the surface has been recognized as a radio- 

 meteorological parameter of some importance 



4 Data from these two publications (published by the National 

 Weather Records Center, Asheville. N.C., under the sponsorship of 

 the World Meteorological Organization [WMO] and the U.S. Weather 

 Bureau) will hereafter be referred to as "weather summary data." 



[Saxton, 1951 ; Bean and Meaney, 1955 ; CCIR, 

 1965]. Therefore, maps are included of the 

 mean value of this parameter for all 12 months, 

 with a more comprehensive network of stations 

 than was available from the complete radio- 

 sonde data sample discussed above. For this 

 purpose a sample of 268 stations was chosen 

 from those available in the Monthly Climatic 

 Data for the World and the National Summary 

 of Climatological Data (U.S.A.). 4 These pub- 

 lications list monthly mean values of pressure, 

 temperature, and relative humidity or dew point 

 for the surface and some of the mandatory pres- 

 sure levels from radiosonde data. For stations 

 near sea level, the 900-mb level is very close to 

 1 km above the surface, but for most stations 

 outside of the U.S.A. the 850-mb level was the 

 lowest reported ; the altitude of this pressure 

 level varies between roughly 1.3 and 1.5 km 

 above sea level. However, it was felt that inter- 

 polation from the 850-mb data, using separate 

 exponentials for the wet and dry terms, would 

 yield fairly accurate values of monthly mean N 

 at the required 1-km height. 



The radiosonde-derived data described in the 

 preceding paragraphs constitute the basic data 

 from which this atlas has been prepared. In the 

 following sections the methods of reduction and 

 presentation of these data are discussed, as well 

 as the consistency and reliability of the results 

 so obtained. 



