WORLD MAPS OF EXTREME iV-GRADIENTS 17 



5.5. Reliability and Limitations of 

 Ground-Based Refractivity Gradient Data 



Because ground-based gradients are so sen- 

 sitive to local effects, such as terrain and land- 

 water relationships, it was impossible to contour 

 figures C-l through C-56 for individual small 

 areas. For instance, although Madrid (on the 

 high interior plateau of Spain) experiences 

 little ducting during the year, it is surrounded 



by areas of high ducting incidence, and no at- 

 tempt was made to delineate this small region 

 of nonducting. Also, refractivity gradients cal- 

 culated from radiosonde observation levels sep- 

 arated by less than 20 m may be seriously affect- 

 ed by instrumental errors, so ground-based lay- 

 ers less than 20 m thick were not included in 

 the analysis. Consequently, very shallow duct- 

 ing (such as that found at certain times over 

 oceans, under dense jungle canopies, and in 



x 

 o 



-20 -30 -40 -50 -60 -70 



N-GRADIENT 



-100 



-110 



-120 



Figure 6. Five-year mean vertical refractive gradient profile: Nicosia. 



mountain valleys) is not included in the con- 

 toured data; however, such layers may be in- 

 tense enough to create trapping conditions for 

 frequencies down to 600 Mc/s [Jeske, 1964; 

 Baynton et al., 1965 ; Behn and Duffee, 1965]. 



The time of day represented by the available 

 observational data must also be considered for 

 any variable which has a definite diurnal trend. 

 Therefore, for a true comparison of worldwide 

 gradient behavior, it would be desirable to use 

 comparable data recorded at least twice a day 

 at standard local or sun-referenced time. How- 



ever, because simultaneous data are needed for 

 the preparation of synoptic maps, all stations 

 in the U.S.A. and many in the European coun- 

 tries schedule radiosonde observations at 0000 

 GMT and 1200 GMT. Many stations in other 

 parts of the world take only one observation per 

 day (usually at either 0000 GMT or 1200 GMT, 

 but there are exceptions, e.g., 0600 GMT at 

 Abidjan, Dakar, and Niamey). Even if all sta- 

 tions had a common GMT hour for taking ob- 

 servations, the diurnal problem would still exist 

 because the local time for any designated GMT 



