RADAR STORM OBSERVATION 
ditions as shown by an RHI scope. This spectacular 
phenomenon is best detected when the precipitation is 
due to convective causes as was the case when the 
photograph shown in Fig. 8 was obtained. Where the 
Fic. 8—-Photograph of RHI scope showing tall (snow) 
shower with abrupt shear below the altitude of 9000 ft. Lack 
of shear in showers more distant from the radar can best be 
explained by nonuniformity of the wind field. Storms are ap- 
proaching the radar; shear therefore indicates decreasing 
wind velocity with altitude (in the plane of the picture) up to 
the point where echo signal edges become nearly vertical. 
Actual slope of shower in shear zone about 80° from the verti- 
eal (X-band radar). (W.I.7. Weather Radar Research.) 
precipitation echo signal is vertical, the cell is embedded 
in air moying with constant velocity and direction. 
Where the echo signal slopes, the precipitation par- 
ticles are descending into air moving at a different 
velocity along the direction of the radar beam. This 
difference may be due to a velocity differential with 
height, a direction differential, or a combination of 
both. The slope of the precipitation is a function of 
the fall velocities of the particles giving the echo and 
the wind velocities and directions involved. Unless the 
fall velocity is known, only relative wind velocities and 
directions may be calculated from observations of this 
type [21, 41]. In Fig. 8 it will be noted that in the actual 
storm the trajectory of the particles was almost hori- 
zontal in the shear zone; the 10:1 vertical expansion 
increases the slope on the scope by this factor. 
With increasing use of narrow-beam radars for 
weather observation, hitherto unsuspected phenomena 
have been observed in conjunction with warm-front 
precipitation. Precipitation is occasionally observed to 
form in narrow, closely spaced parallel bands as shown 
in Fig. 9. The formation is suggestive of billow clouds 
formed by shear waves, and is in all likelihood partially 
due to the same action as that which causes clouds of 
this type. The phenomena are rather transient, usually 
persisting for only ten or fifteen minutes in any par- 
ticular area. Observation of this occurrence usually re- 
1273 
quires narrow beam width radars (less than 2°) and 
careful adjustment of antenna elevation angle and gain. 
Also, rather short pulse lengths must be employed for 
best presentation. It is interesting to note that perfectly 
Fic. 9.—Photograph of PPI scope showing striking for- 
mation of precipitation echoes formed in parallel bands. 
Radial spokes are ‘‘shadows”’ cast by buildings and chimneys 
near the radar. Bands are suggestive of billow clouds, but are 
actually rain showers oriented north-south. Note the even, 
light rain echo signal which occupies the northern half of the 
scope. (1220 EST, 12/7/49, X-band radar, range 120 miles, 
10-mile markers.) (M@.I.7'. Weather Radar Research.) 
uniform precipitation conditions are seen, though rarely 
over any extended area. Finer and finer detail becomes 
evident as the resolving power of radars is increased by 
employment of narrower beam widths. It may be that 
reasonably uniform rain can extend only over an area 
of a few tens or hundreds of square meters. 
Another phenomenon observed in conjunction with 
warm-front precipitation (and also under other condi- 
tions), is the descriptively designated ‘“‘bright band.” 
It receives this name from its appearance on RHI 
scopes; a typical example is illustrated in Fig. 10. Fig- 
ure 11 shows its appearance on the R scope of a radar 
with a vertically directed antenna. It may be seen that 
the bright band is an approximately horizontal layer of 
exceptionally strong echo signal. The RHI scope has 
shown the following conditions to exist during bright- 
band detection: 
1. The band may be the only precipitation return 
detected by the radar. 
2. Precipitation may be seen below the band with no 
return above (and occasionally vice versa). 
3. The bright band may be accompanied by precipi- 
tation echo signals both above and beneath it. 
While certain detailed processes of bright-band for- 
mation are the cause of dispute, all investigators agree 
that it is in some way connected with the change of 
state which occurs at the freezing level. The band is 
invariably observed nearly at or slightly below this 
