174 SYNOPTIC RADIO METEOROLOGY 



estimate of refraction errors and the introduction of appropriate correc- 

 tions for radio range and elevation angle errors for radio navigational 

 equipment. 



VHF-UHF radio field strengths beyond the normal radio horizon will 

 also differ from air mass to air mass. It has been known for many years 

 that the seasonal cycle of VHF radio field strengths received far beyond 

 the normal radio horizon were correlated with the refractive index [7, 8, 

 9, 10] and that significant changes in field strength level are observed from 

 air mass to air mass [11, 12, 13.] Speaking about signal level on a 60 

 Mc/s beyond the horizon radio path near Boston, Mass., Hull [11] states 

 that during the winter low signal levels prevail during the presence over 

 the path of fresh polar air. Periods of high signal occur when a cold, dry 

 polar air mass is overrun by warm, moist air of tropical maritime origin. 

 Hull's analysis represents early recognition of refraction and reflection 

 phenomena on a synoptic scale. Later work on seasonal changes of fields 

 and A'^ represents, in a way, a summary of synoptic conditions over a 

 period of time. 



Gerson [2] was one of the first to consider the variation of the radio 

 refractive index, n, in terms of seasonal and air mass changes. Gerson 

 divided n into two parts, one density-sensitive and the other moisture- 

 sensitive. This division is equivalent to the wet- and dry-term separation 

 of A'^ = (n — 1) 10^ in chapter 4, section 1. Gerson was able to measure 

 seasonal thermal changes by variation in the dry term and seasonal 

 moisture changes by observed variation in the wet term. Gerson pre- 

 pared graphs showing a sinusoidal variation of the dry term with a warm 

 season trough and a cool season crest, indicating density changes in inverse 

 proportion to the temperature. The wet-term component of n, on the 

 other hand, was observed to attain its maximum during the warm season 

 when the dry term was at its minimum. In arctic and antarctic locales, 

 the surface variation of the wet term was found to be quite small while in 

 temperate and tropical climates there was a sizable annual variation of 

 the moisture component. As a pertinent aside, Yerg [14] showed that 

 even during the long, cold, arctic night, vertical variations in moisture 

 made significant contributions to the A'' profile. Apparent ducting 

 gradients, obtained by neglecting the wet term at low temperatures may, 

 in actuality, be only slightly more refractive than standard. 



Continuing his investigation, Gerson next turned to the analysis of 

 refractive index changes within various air masses. Using air mass data 

 available in the meteorological literature of the day, he charted mean 

 refractive index profiles for different air mass types. The largest initial 

 values and also the largest vertical gradients of n occurred with tropical 

 maritime air. The air mass with the weakest gradients and, therefore, 

 the poorest refraction properties was found to be the polar continental 

 type. Common to all of Gerson's air mass refractive index graphs is an 



