390 
5 10 15 20 
PRECIPITATION 
INCHES 
Fie. 3—Number of unofficial network hail days 
vs. precipitation totals for period April 1 through 
October 31 
TaBLe 1—Area frequency versus point frequency 
of hail 
re eee es 
1949 33 4 8:1 
1950 23 3 8:1 
1951 34 9 4:1 
1952 16 2 8:1 
1953 21 a Sel 
1954 9 4 P81 
1955 26 9 S35 
1956 13 3 4:1 
1957 25 6 4:1 
1958 25 4 6:1 
Totals 225 51 4.4:18 
* Ten year average. 
TaBLeE 2—Hail-thunderstorm ratio 
Item = =2 | Ratio 
S@ |soa 
ou |SuUU 
& |s 
10-year unofficial area 225 | 391 | 1:1.7 
10-year official point 51 | 391 | 1:38 
59-year official point 4} 43) 1:11 
W. BOYNTON BECKWITH 
after June when the frontal contributions to 
shower development decrease in potency. The 
investigations by Douglas and Hitschfeld [1958] 
in Alberta and Stout and others [1959] in Ilb- 
nois suggest that peaking of hail activity is 
somewhat later in the season in Canada and at 
about the same time in Illinois. However, it 
should be noted from Figure 4 that peaking of 
hail days is spread quite broadly on a year-to- 
year analysis. 
Time of initial fall of hail in the network por- 
trayed in Figure 5 shows a peaking of about a 
third of the reports between 14h 00m and 16h 
00 MST. Two-thirds of the time, hail fall com- 
mences in the five-hour period between 13h 00m 
and 18h 00 MST. This pattern demonstrates the 
powerful effects of the thermals generated in 
the mountains to the west of the network. If the 
broad scale upslope conditions which are asso- 
ciated with half of the hail developments in this 
study were the major lifting force in triggering 
hail generating thunderstorms, we might look 
for a flatter distribution curve. It is well rec- 
ognized by forecasters, for example, that the 
general upslope circulation that produces nearly 
all of the cold weather snows, is equally effective 
at night as during the time of day when thermal 
effects are felt. 
Hail size—The frequency distribution of hail- 
stone size is portrayed in Figure 6. Hailstones of 
one-inch diameter can inflict considerable dam- 
age not only at the ground, but also to aircraft 
in flight. It is to be noted that an average of 
seven reports of stones of this size or larger are 
collected in the Denver network each year. Of 
more importance is the consideration of the 
greater frequency of occurrence of these larger 
stones at aircraft operating altitudes before the 
effect of melt or sublimation has taken over in 
the fall from point of origin to the surface. 
Geographically, hail size frequency at the 
northern end of the hail belt in Canada bears a 
close relation to the figures, according to data 
compiled by Douglas and Beckwith [1958]. In 
the Middle West, the smaller size stones appear 
with greater frequency, judging from figures 
published by Stout and others [1959]. The sea- 
sonal change in hail size expectancy is shown in 
Figure 7, depicting Denver and Alberta [Douglas 
and Beckwith, 1958] experience. It will be noted 
in general that in months of relatively low levels 
of activity, maximum stone size is small. 
Another relationship between hail size and ac- 
tivity level which has been borne out in the 
