Analysis of Hailstorms in the Denver Network, 
1949-1958 
W. Boynton BEckKwITtH 
Meteorology Department, United Air Lines, Inc., Denver, Colorado 
Abstract—Data collected during 225 hail days over the past ten years are investi- 
gated. The synoptic and thermodynamic conditions associated with hail development 
are analyzed and comparisons are made between storms of this area and those of the 
Middle West and Alberta. Upper wind distribution and the position of the tropopause 
are related to the development and growth of hailstorm-producing cells. 
Introduction—Since 1949 a cooperative hail 
reporting network in and around the city of 
Denver has made possible the collection of suf- 
ficient detailed hailstorm data to make the be- 
ginnings of a climatology of this troublesome 
hydrometeor. Organized first to learn more about 
the true areal frequencies of hailstorms in this 
vicinity, and to establish new forecasting tech- 
niques on the basis of the various parameters 
that can be measured with a micro network, the 
project has been a continuing one since then. 
This paper is an extension of earlier studies 
[Harrison and Beckwith, 1950; Beckwith, 1956; 
Douglas and Beckwith, 1958] with respect to 
hailstorm characteristics and some thermody- 
namie relationships. Comparisons between the 
Colorado hailstorms and those of the Midwest 
and Canada are made on the basis of studies by 
Stout and others [1959], Douglas and Hitschfeld 
[1958], Fawbush and Miller [1953] and others. 
Since 1953 an AVQ-10 radar has been used to 
monitor with a PPI display, some of the heavier 
hailstorm developments in the Denver area. 
Some results of this phase of the investigation 
have been published earlier [Harrison and Post, 
1954; Beckwith, 1956]. The relation of the net- 
work to the Front Range of the Rockies is shown 
in Figure 1. The area covered by the cooperative 
network is shown in Figure 2. Site of the United 
Air Lines radar corresponds very closely to the 
position labeled USWB. The actual number of re- 
porting stations shown in Figure 2 has varied 
from 12 in 1949 to 30 in 1951 to an average of 50 
each year since that time. 
Time, space, and frequency variations—It has 
frequently been pointed out that a more correct 
picture of the incidence of hail for a given area 
is afforded by network reporting than by point 
reporting. Hail frequency for the Denver area 
is shown in Table 1 and is expressed as a ratio 
to the official point reporting of the Weather 
Bureau. The Denver network encompasses about 
150 sq mi. Even in such a small area as many as 
five hailstorms have been reported in one day. 
The 225 hail days shown in the ten-year total 
represent about 300 individual hailstorms. 
Many earlier studies have expressed the fre- 
quency of hail occurrences in ratio to thunder- 
storm days. This is a useful relation for point 
summaries, but when area figures are summa- 
rized some bias is introduced if one reporting 
station is the basis for thunderstorm day counts. 
The hail-thunderstorm ratio of Table 2 has been 
subjected to this bias. Also, the year-to-year 
ratio has varied widely during the past ten 
years, a function of the change in hail patterns 
from one year to the next. An opinion expressed 
very often by forecasters concerned with the 
problems of the ‘hail belt’ as well as by trans- 
port pilots frequenting this region is that nearly 
every thunderstorm developing off the Rocky 
Mountains contains hail in some stage of its 
development. 
A relationship that has proven more valid 
than the hail-thunderstorm ratio during the past 
ten years is shown in Figure 3. For the period 
April through October, precipitation is almost 
entirely from shower activity, the exceptions be- 
ing frontal and upslope developments which are 
of some importance in April and October. Since 
this precipitation is shower-generated, a good 
indicator for hail activity would be expected. 
The precipitation for 1957 which falls away 
sharply from an otherwise good scattering, re- 
fleets the two very wet months of April and May 
of this year which were characterized by a late 
heavy snow and a series of cloudbursts. 
The sharp rise in hail activity in the Denver 
area in late spring is well illustrated in Figure 4. 
On the average, activity falls off more gradually 
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