PERCENT OF STORMS IN WHICH EVENT OCCURRED 
DONALDSON, CHMELA, AND SHACKFORD 
i 
NN 
Ea L100] ! 60 
I wt 
TORNADO OR FUNNEL ALOFT. 
Fic. 5—Frequency of storm events in hail repeater storms during 20-min 
periods following first report of hail 
of first hail than to time of first echo, since many 
first echoes were not observed at all in our sched- 
ule of radar observations and only nine of them 
could be stated within an accuracy of five minutes 
and 13 within 11 min. 
Figure 5 depicts the hail and severe damaging 
windstorm events, including tornadoes, during 
the time following first hail in the hail repeaters. 
Note the tendency toward a 100- to 120-min 
periodicity in tornado activity and occurrence of 
large hail. This is an integrated picture; the seven 
individual storms in which such changes are ob- 
served show a periodicity of 30 to 70 min with, 
at most, three periods apparent. 
Figure 6 shows the time variation of maximum 
echo height in all 20 storms. The first echoes are 
arbitrarily started at a height of 15,000 ft with a 
maximum rate of height increase of 2000 ft/min. 
There is a wide variation in time between first 
echo and first hail (14 to more than 162 min), in 
maximum height reached by an echo (27,000 to 
56,000 ft), and in total height interval covered 
by the echo tops (always greater than 20,000 ft). 
Wide variety in echo-top behavior is the rule. 
Nevertheless, some general trends superimposed 
on these large variations are brought out by Fig- 
ure 7, which shows the median and maximum 
echo-top history of the two storm groups. 
The echo tops of hail repeaters differ from 
those of the single-hail producers in the following 
particulars: They are rising when the first hail 
falls, they maintain extreme heights for greater 
periods of time, they attain a higher over-all max- 
imum, and they have a longer lifetime. These 
characteristics are similar in certain respects to 
those discovered by Douglas and Hitschfeld in 
their sustained hail producers. 
The most intense radar echo found anywhere 
in the storm was observed at various times during 
the life cycle of 17 of the hailstorms, in the man- 
ner described by Donaldson [1958]. The height of 
the most intense part of these echoes ranged from 
near the surface up to 31,000 ft, with half of the 
observations centered between 13,000 and 24,000 
ft. At least part of the reason for the surprisingly 
high altitude of the most intense radar echo in 
hailstorms is due to the high attenuation in heavy 
rain of the 3.2-em waves radiated by the CPS-9 
radar. Attenuation by ice is much lower than by 
the same mass of rain. Other possible causes in- 
clude high particle concentrations accumulated 
near the maximum of a non-steady-state updraft 
