1272 
from which it may be seen that at a range of 200 miles 
the lowest portion of a horizontally directed beam of a 
radar located at sea level is about 20,000 ft above sea 
level [2]. 
One of the finest practical applications of radar storm 
detection is made when radar-equipped aircraft use it 
to avoid violent convective activity while flying through 
an active cold front [11, 46]. As Fig. 6 shows, the front 
is far from bemg a solid mass of storms of moderate 
altitudes. This application is especially useful at night 
or when the pilot’s vision is restricted by clouds. A re- 
cent development has been shown to have value in selec- 
tion of the least turbulent portions of storms when it is 
impossible to avoid them completely. This will be dis- 
sussed in the section concerning echo-signal analysis. 
Radar observations of the approach of a cold front 
show the following sequence of events [67]: Scattered 
storm-echo signals are first detected at maximum ranges 
(100-300 miles) depending upon the activity of the 
front. These echoes are caused by hydrometeors in the 
upper portions of the tallest cumulonimbus along the 
front, and generally lie in an are which may be closely 
identified with the position of the front as reported by 
surface observation stations. As the front approaches, 
the radar detects precipitation at successively lower 
levels and the original cells appear to increase in size 
and intensity. When the nearest portion of the front is 
about 50 miles away, a large part of it appears to be a 
solid line of precipitation if the antenna elevation angle 
is kept at or near zero degrees. The inexperienced ob- 
server will sometimes conclude that the front is actually 
intensifying, whereas the radar is simply detecting rain 
at lower levels which is almost invariably more wide- 
spread. This trend continues until the front passes over 
the radar, at which time echoes from the more distant 
storms along the front may disappear from the scope 
entirely because of rain attenuation. At this time the 
precipitation may appear to be almost evenly dis- 
tributed around the radar for a distance of many miles. 
After frontal passage, the above sequence of events 
is reversed until the front passes beyond the maximum 
range of detection or dissipates. 
While the Ime of storms taken as a whole appears to 
approach the radar, close examination reveals that the 
individual cells have a component of motion in the 
direction of the warm air movement ahead of and over 
the front. Thus, if the front appears to approach from 
the northwest and the warm air movement is from the 
southwest, the cells will show a movement just about 
due eastward, depending on the relative velocities of 
the cold and warm air masses. 
2. Warm front. Interpretation of radar displays re- 
sultmg from warm-front precipitation is considerably 
more difficult than the interpretation of echoes from 
cold-front precipitation. This is a result of the larger 
area covered by the precipitation and the possible vari- 
ation of conditions. The precipitation causing the echo 
signals exhibits varying degrees of convective activity 
depending upon stability and convective stability con- 
ditions in the air masses involved. When conditions are 
stable in both air masses, the return appears as shown 
RADIOMETEOROLOGY 
in Fig. 7. Durmg more unstable warm-frontal condi- 
tions the PPI scope will show stronger echoes from in- 
dividual storm cells. Absence of uniform or systematic 
Fie. 7—Stable warm-frontal precipitation echo signals on 
a PPI scope. Uniform snow echo signals are present at all 
azimuths. (1010 EST, 2/20/47, range 20 miles, 5-mile markers.) 
(M.I.T. Weather Radar Research.) 
patterns 1s frequently observed. However, some sort of 
cellular structure is nearly always observed, and deter- 
mination of the velocity and direction of cellular move- 
ment is usually straightforward. The movement of pre- 
cipitation cells seems to be controlled by several factors, 
among which are the circulation within the system and 
the movement of the entire system itself. Attempts have 
been made to relate precipitation-cell movements to 
the winds aloft, but only rather general correlations 
have been found, and sometimes even these fail com- 
pletely [29]. It is entirely probable that there is no 
simple relationship between the winds aloft and cell 
movement which can be applied to all storms. In order 
for a given precipitation cell to maintain itself it must 
have a continuous supply of moisture. This implies that 
air moves through the system at some speed different 
from that of the system itself. 
At times it has been noted that the precipitation 
nearest to the radar is rain, while that detected at 
greater distances must be in the form of snow because 
of the increase in height of the beam with range (see 
equation (6)). As yet, no technique has been developed 
for positive determination of the nature of the precipi- 
tation, except in special cases, for example, when the 
height of the freezing level is known. The appearance 
of rain and snow echo signals on the PPI scope may be 
identical when both are present. However, differentia- 
tion is sometimes possible by R-scope inspection, as 
shown in Fig. 11. 
The approximate height of the frontal surface may 
sometimes be determined by the presence of shear con- 
a 
