1274 
level as determined by radiosonde and aircraft (and 
sometimes surface) observation [22, 25, 26]. The ob- 
served thickness of the band can never be less than the 
Fic. 10—Photograph of RHI scope showing horizontal 
layer of strong echo signal (bright band) at altitude of about 
11,000 ft. Tall vertical showers are indicative of convective 
activity. (1930 EST, 9/25/47, X-band radar.) (M.J.7. Weather 
Radar Research.) 
pulse resolution of the radar used to observe it, and 
since it has occasionally been determined to be no more 
than this amount for a given radar system, there 1s evi- 
ALTITUDE (MILES) 
BRIGHT 
BAND? 
RAIN 
ECHO 
Be ECHO INTENSITY pee 
Fie. 11.—Photograph illustrating appearance of R scope 
when antenna is directed vertically and echoes are received 
from rain and snow. The weaker echo signal of the snow is due 
to its greater range and lower dielectric constant. The echo 
from rain shows much greater intensity variations because of 
the wider range of fall velocities of the water particles. (X- 
band radar, range 5 miles, 1-mile markers.) (V1.7. Weather 
Radar Research.) 
RADIOMETEOROLOGY 
dence that the thickness of the band may vary from as 
little as a few tens of meters to several hundred meters 
in thickness or more. It has been observed in connec- 
tion with thunderstorms after convective activity has 
subsided [10, 19]. Two theories have been advanced to 
explain this phenomenon: 
1. The region of strong echo is due to drop formation 
in the colloidally unstable layer of heterogeneous ice- 
water mixture. Any precipitation detected above that 
altitude would therefore probably be caused by con- 
vective transport [19]. 
2. Snow particles, too small to give more than a weak 
echo, fall to the zero degree isotherm, and melt at or 
just below this level. While melting, they have the low 
fall velocity of snowflakes, but the high reflectivity of 
water. Coalescence, often observed near melting tem- 
peratures, also serves to increase the reflectivity. After 
the snowflakes have completely melted, the fall velocity 
increases and the drops become widely spaced. The re- 
sult is a region of weaker echo below the level of melt- 
ing [10, 24, 52). 
Most investigators are inclmed to favor the latter 
theory, which the weight of observational evidence 
seems to support. There is little doubt that, in the ab- 
sence of other observations, the bright band provides 
an excellent indication of the approximate height of 
the freezing level. To a lesser extent, it is also an in- 
dicator of the stability of the atmosphere; a thin band 
is indicative of relative stability, and a thick one (or 
the absence of a band) suggests convective activity 
near the freezing level [10]. At the present time, how- 
ever, this is only a rough qualitative measure. 
Under some conditions, the warm front provides the 
closest approximation to uniform precipitation rates 
which are so necessary at present for measurements 
made to study the relationship between rainfall in- 
tensity and the power of the echo signal. Efforts are 
being made to monitor the received power of storm 
echo signals continuously on two different frequencies 
[12, 42] (see Fig. 16). The radars are directed and 
ranged over a recording rain gage to establish relation- 
ships between rate of rainfall and echo-signal power. 
Best correlations between the two have been found 
during steady rainfall associated with warm-frontal 
conditions. 
3. Occluded front. Little has been published concern- 
ing radar storm detection during occluded frontal con- 
ditions. Because of the complexity of the frontal system 
few generalizations can be made concerning precipita- 
tion types or patterns. Some situations show widespread 
stratified precipitation, others show just as widely 
spread convective cells. An excellent place to study 
fronts of this type would be in Norway or in British 
Columbia, where radars sited near the coast could be 
directed westward over the oceans, and observations 
could be made of fronts unmodified by topography. 
B. Orographic Precipitation. This type of precipita- 
tion, being associated with uneven terrain, is somewhat 
difficult to observe on PPI scopes if the antenna is 
directed horizontally. Increase of the antenna elevation 
angle is necessary to eliminate echo signals from the 
terrain causing the precipitation. There are no unique 
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