VARIATIONS IN RADAR COVERAGE 133 
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 sq 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. 
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 
this 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. 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. 
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- 
ence 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. Superretraction 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. 
