Abstract :
Some features of radio-refractivity which are important from engineering point of view are described with reference to published data on radio-refractivity up to a height of 3 km from the surface of the Earth and radiosonde data on pressure, temperature and humidity up to 4.5 km covering a period of five years. Three broad radio-climatic areas having distinct meteorological and refractivity characteristics are delineated. Correlation between surface refractivity Ns and the difference of refractivity over height-intervals of 1 km and 2 km from the surface, designated as ¿N1 and ¿N2 respectively, is examined. For coastal areas and islands, correlation with either ¿N1 or ¿N1 is found to be poor, while for landbased stations, it is observed to be good. An approximately exponential relationship relates the variation of Ns and ¿N1. Several models for height variation of refractivity are examined and compared with observed structures. A bi-exponential model for inland region and a two-part model consisting of a linear variation up to 1 km or 1.5 km followed by an exponential variation for coastal areas appear to be satisfactory. The long-term transmission loss predicted on the basis of this model and corresponding observed values in case of one trans-horizon radio circuit are found to be in fair agreement.
