SI MMVRY OF BASK. FACTS CONCERNING PROPAGATION AT RADAR FREQUENCIES 



185 



radar they lirst appeared as a nuisance, but more 

 recently they have been put to practical use. In 

 tropical climates they are very helpful for aerial 

 navigation. 



Cloud echoes may be distinguished from other 

 echoes by their fuzzy and diffuse appearance. Not 

 all clouds show up on a scope with equal strength. 

 The strength of the echo seems to depend primarily 

 on the size of the water drops within the cloud or 

 rain storm. Ordinary clouds such as form an even 

 overcast (stratus clouds) are not usually visible on 

 the scopes; the droplets that compose these clouds 

 are so small that they reflect very little energy. 

 Violent showers give intense echoes on the scopes. 

 Storm echoes can be seen much farther than normal 

 land targets, even under standard conditions, because 

 of their great spread in the vertical direction. 



In discussing cloud reflections it must be clearly 

 understood that there is no physical relation between 

 cloud echoes and refraction; the mechanics of duct 

 formation is not related to clouds, and with respect 

 to the bending of radio waves a cloud is merely 

 another airborne target. 



165 SUMMARY OF BASIC FACTS 



CONCERNING PROPAGATION AT 

 RADAR FREQUENCIES 



1. Standard propagation results in a slight down- 

 ward bending of the rays throughout the atmosphere, 

 leading to an increase of the horizon distance com- 

 pared to the geometrical value. It is taken into 

 account operationally by using coverage diagrams 

 with a % earth's radius; on a diagram modified in 

 thjs way the rays appear as straight lines. 



2. Guided propagation occurs almost exclusively 

 in the lowest 2,000 ft above the ground and usually 

 is confined to the lowest few hundred feet (except 

 in warm climates). 



3. Superrefraction resulting in guided propagation 

 or trapping is produced : 



a. By a pronounced decrease of moisture with 

 height (moisture lapse), or 



b. By a pronounced increase in temperature 

 with height (temperature inversion), and 



c. Particularly, by a combination of both of 

 the above conditions. 



4. Of the meteorological conditions conducive to 

 guided propagation or trapping, the most outstand- 

 ing are: 



a. Over sea: flow of warm, dry air over colder 

 water producing temperature inversions and 

 evaporation into the lowest layers. 



b. Over land: nocturnal cooling of the ground 

 with clear skies and calm air or light winds 

 (if moisture distribution is favorable) . 



c. Over both sea and land : low-level subsidence. 



5. Conditions in a barometric high, including calm 

 and clear skies and especially low-level subsidence, 

 favor trapping especially during the night (but do 

 not necessarily produce it). Conditions in a baro- 

 metric low, including strong winds, intense turbu- 

 lence in the lowest layers, and overcast skies are 

 conducive to standard propagation. 



6. When the transmitter is within the duct, radar 

 range is increased for surface targets (ships) and air- 

 craft flying in the duct. At the same time there is an 

 increase in fixed echo strength and consequently in 

 ground clutter on the scopes. This may be accom- 

 panied by a change in the range of detection for 

 craft flying above the duct. 



7. When the transmitter is outside the duct, the 

 range may be either increased or decreased from its 

 standard value. 



8. Effects of nonstandard propagation are negli- 

 gible when the angle of elevation of the target is over 

 1 degree. Failure of detection at such angles must be 

 attributed to other causes. 



