184 



VARIATIONS IN RADAR COVERAGE 



tion in the lowest layers is such as to reinforce or at 

 least not to counteract the effect of the temperature 

 distribution, that is, when the moisture decreases 

 not too slowly with height. Nocturnal cooling is 

 greatest with clear skies and is quite small under 

 an overcast. Hence guided propagation over land 

 occurs at night almost exclusively with clear skies. 

 This type of temperature inversion is strictly confined 

 to land areas. It does not occur over the ocean because 

 the sea temperature does not show appreciable daily 

 variations. Temperature inversions caused by noc- 

 turnal cooling are most pronounced over dry land 

 (desert) but will occur almost anywhere over land 

 with a clear sky and a not too humid atmosphere. 



Subsidence 



Another weather phenomenon favorable to trap- 

 ping is subsidence. By subsidence is meant the slow 

 downward motion, combined with horizontal spread- 

 ing, of air above the lowest layers of the atmosphere. 

 This process, which most frequently occurs in the 

 area of barometric high, will produce temperature 

 inversions; the subsiding air moreover becomes rel- 

 atively much drier than the unaffected air below. In 

 general the subsidence inversion is quite high (e.g., 

 above 4,000 to 5,000 ft). In the light of present 

 knowledge it appears that high subsidence inversions 

 do not generally affect guided propagation when the 

 sets are situated at low altitudes. It appears, how- 

 ever, that such subsidence inversions might materi- 

 ally affect communications or airborne radar search 

 aloft. Lower subsidence inversions (1,000 to 2,500 

 ft) along the southwestern coast of the United States 

 are known to produce stable duct layers affecting 

 radar coverage at low angles. 



Turbulence of the Air 



This has a distinct normalizing effect in that it 

 tends to smooth out the temperature and moisture 

 variations which are conducive to guided propaga- 

 tion. Moderate to strong winds produce a turbulent 

 layer extending normally to a height of about 4,000 

 ft. The air is well mixed within this layer, and 

 consequently the standard type of refraction prevails. 

 Regions of a barometric low are characterized by 

 strong to moderate winds and pronounced turbu- 

 lence in the lower layers. In addition low pressure 

 areas usually have overcast skies. Hence a barometric 

 low will as a rule lead to propagation of the standard 

 type. 



Frequency of Occurrence 



It is extremely difficult to estimate in general 

 terms the frequency of occurrence of guided propa- 

 gation, since statistical data are almost nonexistent 

 at present except for very limited regions in Europe 

 such as the North Sea. In the central Mediterranean 

 during the summer months of 1943, ducts have been 

 observed on 9 days out of 10. Frequent trapping has 

 also been observed in some parts of the Pacific. At 

 other times and places guided propagation might be 

 an unusual occurrence, especially if the barometric 

 pressure is generally low and the winds strong. It 

 seems advisable to consult a weather officer with 

 regard to any given locality. 



Measurements 



In order to determine weather's influence upon 

 radar in a quantitative way, the variation of refrac- 

 tive index with height must be determined. This 

 requires accurate knowledge of the temperature and 

 moisture distribution in the lowest few hundred or 

 thousand feet of the atmosphere. The ordinary 

 radiosonde is not well adapted to measurements of 

 this type because the measured points on an ascent 

 are usually spaced several hundred feet apart. Among 

 the methods which have been developed for this 

 purpose during the past two years, the one most 

 generally adopted uses a captive balloon (or kite) 

 which carries aloft electrical temperature and moist- 

 ure — measuring elements. These are connected to a 

 meter on the ground by means of thin wires attached 

 to the cable holding the balloon. This device permits 

 measurements at intervals as closely spaced as 

 desired. A psy chrome ter held out of the window of a 

 slowly flying plane has been used with good success 

 in the absence of more elaborate equipment. 



16.4 



CLOUD ECHOES IN RADAR 



Cloud echoes (more precisely, precipitation echoes) 

 are observed frequently on radar scopes. At times 

 they have caused confusion by blotting out other 

 targets. Their similarity, upon certain occasions, to 

 actual targets have caused some difficulty in the 

 interpretation of the signals. 



These echoes are caused by a reflection of the radar 

 pulse from the raindrops in the clouds (or in rain 

 storms). The amount of reflection increases very 

 rapidly with frequency. Cloud echoes are quite 

 exceptional below about 1,000 mc. In microwave 



