346 MONTHLY WEATHER REVIEW VOI. 98, NO. 5
UDC 651.678.7:661.601.81: 651.609.324: &51.589.1(773) 1967
CHARACTERISTICS OF HAIL-PRODUCING RADAR ECHOES IN ILLIN
NEIL G. TOWERY and STANLEY A. CHANGNON, JR.
Illinois State Water Survey, Urbana, 111.
ABSTRACT
Data from 103 hail echoes on 24 days in 1967 and 50 no-hail echoes from the same days were analyzed to describe
hailstorm characteristics and to provide information useful in operational detection and forecasting of hail-producing
echoes. Echo characteristics investigated included locations of echo formation and dissipation, echo reflectivities,
echo-top heights, echo duration, direction of motion, speed, time of occurrence, and associated synoptic weather
conditions. A single hail-echo model could not be derived because of the extreme variability found in all character-
istics. However, distinctive echo models could be developed for the three predominant hail-producing synoptic
weather conditions, cold fronts, stationary fronts, and low-pressure centers. The frontal hailstorms were faster
moving, longer lived, and had taller echoes than those with low-pressure systems. Hail production after echo inception
varied from an average of 32 min for low conditions to 59 min for cold frontal echoes. The average hail-echo top
exhibited a 5,000-ft growth in the 15-min period prior to the average time of hail, suggesting that a major updraft
surge was the prime producer of hail. The no-hail echoes occurring on hail days had characteristics of speed, direction
of motion, reflectivity, and location that were very similar to the hail-producing echoes. The only distinct consistent
difference between the hail and no-hail echoes in all synoptic situations wm that the hail-echo tops averaged between
2,000 and 4,000 f t higher throughout their entire durations.
1. INTRODUCTION
Information concerning the behavior of hail-producing
echoes was sought as part of a comprehensive hail research
program in Illinois (Changnon 1969). Knowledge of the
characteristics of both hail-producing and no-hail echoes
has value in two areas. One concerns the identification and
point-area prediction of hailstorms on an operational basis
for aircraft storm avoidance, public warnings, and selec-
tion of approaching storms for seeding in hail suppression
projects. Most prior research on hail-echo identification
has concerned their heights (Douglas 1963) or reflectivity
profiles (Donaldson 1958, Wilk 1961), but recent studies
(Rinehart et al. 1968, Dennis and Musil 1968) have shown
that high-reflectivity characteristics aloft are not well
correlated with surface hail. Information about character-
istics of hail-producing echoes is also quite meaningful for
increasing knowledge of the causes of hail generation.
Results on various hail-echo characteristics including
location, duration, direction of motion, speed, time of
occurrence, reflectivity values, echo-top heights, and
associated synoptic weather conditions were obtained for
103 hail-producing echoes. These results are compared
with those obtained for 50 no-hail echoes. From these
analyses, models of typical Illinois hailstorm echoes are
developed, and results that have meaning for either
hailstorm identification or hailstorm physics are identi-
fied and summarized.
4. DATA AND ANALYTICAL TECHNIQUES
The radar data consisted of PPI photographs taken in
CPS-9 radar operations on 24 days during April-Sep-
tember 1967. The radar was operated with a maximum
range of 80 n.mi. and with automatic sequential antenna-
tilt and receiver gain reductions (gain step). A photo-
graph was taken at each tilt angle and gain step. The
surface reports of hail and no-hail came from a network
of 1,380 cooperative hail observers and two smaller
networks of 65 raingage-hailpad sites in an 18,000-sq mi
area of central Illinois (fig. 1). These sources provided a
total of 352 observer reports of hail time, 271 observer
reports of no-hail, and 130 hail-time occurrences from the
raingage-hailpad sites for the 24 days studied.
Initially, the analytical procedure consisted of making
a track of each hail-producing echo by plotting the
location of the hail, finding the echo on the film that
corresponded, and plotting its location as far back in time
(prior to hail) and as far forward in time (after hail) as
possible. The plots were of the centroid of the echo on
every 0 tilt photograph (available approximately once
every 10 min) as depicted on a medium level of receiver
gain. The line connecting the centroid positions became the
echo track for its entire duration. The tracks of 50 ran-
domly chosen no-hail echoes were determined in much the
same way, except that the track was started from the echo
formation time and continued for approximately 1 hr.
The medium gain-step level chosen for echo definition
was normally the one midway between maximum sensi-
tivity level and that level where all echoes were eliminated.
This generally was in the 20-28 dB (decibel) range of
reduction from maximum sensitivity.
Table 1 gives the number of hail-echo tracks that
occurred with each synoptic situation, classified according
to their formation and dissipation locations. The location
of the first identifiable appearance (beginning) and the
last identifiable appearance (ending) was determined for
all echoes. However, the actual formation and/or dissipa-
tion locations of about half of the echoes could not be
May 1970 Neil G. Towery and Stanley A. Changnon, Jr. 341
"$Y Kankakee
Peoria
/.
SCALE OF NAUTICAL MILES I
/
20 40
a
4 n
z
z -
FIGURE 1.-Areas from which 1967 hail data were collected.
TABLE 1.-Frequency of haibecho tracks by synoptic weather classi$cations
Number of tracks
Cold Stationary 'Low Air Total front front maSs
Echo characteristic
Formed and dissipated in range- - .___..____ 5 7 21 2 35
Formed in range, but did not dissipate in
range _______________.____---...----..----- 5 9 1 0 1 5
Dissipated in range, but did not form in
range.. __ __. _____. _____. .____. . _.._.._. - .. 9 2 1 1 7
Neither Iormed nor dissipated in range. - -.. 4 2 1 8 3 3 6
Total ____________________-----------.- 19 46 32 6 103
5
established because they formed or dissipated beyond
the maximum radar range or within a large echo mass. -. - . __ -_ .- _. The totals in table 1 reveal that 50 echoes formed in range,
52 echoes dissipated in range, and 35 of these had known .. x x .
. .. .. tormation and dissipation points.
In addition to the 103 hail-echo tracks, 50 randomly
chosen no-hail echoes were tracked, each of which formed
in range. Sixty percent of the no-hail echoes passed over
10 or more volunteer observers reporting no hail, and all
passed over four or more observers reporting no hail.
These no-hail echoes were chosen from the 14 days on
which the 50 hail echoes (formed in range) occurred, and
the number from each day was made proportional to the
number of hail echoes on that day.
I n the analyses, the data for most hailstorm character-
istics were ranked and divided into thirds, with each third
considered to be a class of that characteristic.
TABLE 2.-Total duration and duration f r o m formation to first hail for hail-producing echoes
Duration, Total echo formation to % of total Cumulative duration, min % of total Cumulative hail, min % (35 echoes) % (50 echoes)
0-19 18 18
20-39 38 66
4&69 22 78
80-79 8 86
80-99 8 04
2100 6 100
Median = 3 7 d n
Average= 44 min
Shortest= 6min
bngest=l52 min
20- 39 14 14
40- 69 17 31
60- 79 14 45
80- <99 23 88
100-119 23 91
2120 9 100
Median = 80min
Average= E4dn
Shortest= 30mIn
bngest=197 rnin
The seven synoptic situations sampled in this study
were grouped into four categories. The fist category was
the cold front, and the second was the stationary frontal
category which included the few warm frontal cases. The
few cases with closed Lows or troughs at the surface and
aloft were included in the low category. The fourth
category was air mass, but certain analyses of this category
were limited by the small sample (table 1).
3. LOCATION
Possible preferred areas of formation and dissipation
of hailstorms were studied from map plots of echo tracks.
Since the sample was relatively small for the area involved,
the echo formations and dissipations were grouped and
studied according to their occurrence in six 60" sectors.
The analyses revealed that the northwest sector (270'-
330" ezimuth) was a slightly preferred area for individual
echo formation, and the northern sector (33OO-030'
azimuth) was a preferred area for dissipation or endings.
An analysis of formation and dissipation areas for the
hail echoes when sampled by synoptic causes, time of
day, direction of motion, and duration did not reveal
any preferred areas.
4. DURATION
The aspects of echo duration are summarized in table 2.
More than half of the 50 echoes that formed in range
produced hail in less than 40 min after formation (first
detected on Oo tilt), and the average time was 44 min.
More than two-thirds of the 35 echoes that both formed
and dissipated in range had a total duration of less than
99 min, and the average duration was 84 min.
A comparison of direction of movement and echo dura-
tion before hail for the 50 echoes that formed in range
revealed that northeast-moving echoes had moderate
(30-49 min) to long (2 5 0 min) durations prior to hail
with one-half having long durations prior to hail. The
echoes moving southeast and east-southeast had a
tendency for short (1 2 9 min) durations prior to the hail
with 56 percent producing hail within 29 min after
formation.
340 MONTHLY WEATHER REVIEW Vol. 98, No. 5
TABLE 3.--Comparison of echo duration from formation to jirst hail with average echo speed and time of day TABLE 4.-Average speed (kt) for the three speed categories during each phase of echo duration
Percent of total for each duration class
Short Moderate Long 529mln 30-49min ISOmin
Characterlstics
speed:
Slow, 519 kt __________ ___
~
____ ~.
~
____ ____ __ ____ 42
Moderate, 20-29 kt _______ __ ..................... 68 78 42
Fast, 230 kt _____ __ ____ ___
~
_____
~
__ ____ _________ 0 16
Morning (0032-1159) _____.____.______.__-.
~
______ 35 31 15
6 21
11
21
24
Time of day:
Afternoon (1200-1759) .... __ - ..._____. _____ .___._ 65 63 64
Evening (18W-2359) _____. . -. . -. __ __ _.____ __. -. __ 0
A comparison of echo durations prior to hail with av-
erage echo speeds is shown in table 3. Moderate speeds
dominated the moderate and short durations, and there
was an equal occurrence of moderate and slow speeds in
the long-duration class.
The comparisons of durations prior to hail and time of
day (table 3) reveal that the morning storms had a
tendency to produce hail quickly. The evening storms
showed no short durations, and a greater frequency of long
durations than of moderate durations. For echoes that
both formed and dissipated in range, there was no rela-
tionship between time of dag and duration.
5. DIRECTION OF MOTION
A comparison of hail-echo speed against direction of
motion revealed that 89 percent of the southeast- and
east-southeast-moving echoes moved faster than 19 k t with
70 percent in the 20- to 29-kt class. The north- and
northeast-moving echoes moved most frequently at
slower speeds with 50 percent between 5 and 19 kt.
Echoes moving east-northeast and east had slow-to-
moderate speeds averaging 24 kt. The predominant echo
direction in the morning was east-southeast, that in the
afternoon was toward the northeast, and that in the
evening was to the southeast.
6. SPEED
The everage speed over the entire track of all the echoes
was 24 kt, and this agrees with the movement of lines of
thunderstorms given by Changnon (196Qa). Average
speeds for the periods before, during, and after hail were
23.6, 24.6, and 24.1 kt, respectively. The average speeds
for echoes in the morning, afternoon, and evening were
26.6, 21.9, and 30.8 kt, respectively. The higher speeds
in the evening were related to the fact that 75 percent of
the evening echoes were cold frontal, and this category
produced the fastest moving echoes. The average speeds
of the three (bast, moderate, slow) categories of speed in
the periods before, during, and after hail are shown in
table 4.
7. TIME OF DAY
The morning, afternoon, and evening storms comprise
24, 64, and 12 percent, respectively, of the 103 echoes.
The decided preference for afternoon occurrence agrees
with resdts for surface hail studies (Changnon 1969).
Speed class Bsfore hail Duringhail After hail
Fast, 230 kt ___________.____________
~
_______ __ 39.6 39.0 35.8
Moderate, 20-29 kt ________ __ ____ __ ____ __ _.__ __ 23.0 22.9 23.8
15.1 15.2 16.3 Slow, 519 kt __________ ______. _____
~
______ ___.
Comparison of the t h e of echo occurrence and the fre-
quency of echo turn prior to hail according to left turn,
right turn, or no turn was made. This was achieved by
drawing a line from the beginning location of the hail
echo through the echo location just prior to hail, and then
another line from this priorlocation through the first known
location after hail. The angle formed by these two lines
was the echo turn. This analysis revealed that 50 percent
of the evening storms had right turns, 17 percent had no
turn, and 33 percent had left turns. With morning storms,
44 percent turned left, 32 percent had no turn, md 24
percent turned right. Afternoon storms showed no distinct
preference for turning.
8. REFLECTIVITY
The average equivalent radar reflectivity factor (here-
after called reflectivity) at formation of the 50 hail echoes
that formed in range was 6.1X102mm6m--3, and the
average reflectivity at dissipation for the 52 hail echoes
which dissipated in range was 1.62X103 mm6 m-3. The
average reflectivity at formation for the 50 no-hail
echoes was 5.5X102 mme m-3, which is very close to the
hail-echo reflectivity at formation. The average reflectivity
at or within 5 min of hail time for the 103 hail echoes was
2X lo5 mm6 m-3. The average reflectivity of the no-hail
echoes at 44 min after their formation (the average time
of hail after formation for the hail echoes) was about an
order of magnitude lower, 1.6 X lo4 mms m-3.
The average range from the radar to the formation
point for echoes (which formed in range) was 51 n.mi.,
that to the dissipation point was 46 n.mi., and that to the
hail location for all 103 hail echoes was 43 n.mi. Thus,
range differences were not great and could not markedly
affect the differences between the average reflectivities.
9. ECHO-TQB HEIGHTS
The analysis of echo heights was performed for the 50
hail and 50 no-hail echoes that formed in range. The radar
was operated with variable sequential tilting of the an-
tenna up to 8 O , and this made it possible t o calculate most
echo-top heights during the echo duration. A plot of all
hail echo-top heights against time after formation was
used to prepare 90 percent envelope curves and an average
height curve (fig. 2). The range of echo heights measured
a t any time is quit,e large, but tends to become smaller
with time. The relatively rapid growth in the average
echo height in the 10-15 min prior to the average hail
time (at 44 min) and the decline in the ensuing 10 min
suggest a strong convective surge during hail growth, and
then a loss of strong updrafts immediately after hail.
May 1970 Neil G. Towery and Stc
60
50
L Y Y
0 40
z
2 30
z 4
I I-
+ I
E 20
I
10
0
0 10 20 30 40 50 60 70
TIME AFTER ECHO FORMATION, MINUTES
0 I c-
; 25
2 k?
8
15.
FIGURE 2.-Average height curve and 90 percent envelope cxves
(made by upper and lower bands) for hail echoes for time after
echo formation. Ten-minute intervals after formation were used
to prepare the curves.
- LOW -
20:
I
Figure 3 is a time-height graph based on average echo
height for three weather categories. To allow comparison
of echo-top behavior between echoes of varying duration,
the heights were determined for the 0 (formation), 25,
50, 75, and 100 percent (hail time) points in time from
formation to the time of hail. Hail-producing echoes
occurring with the three synoptic conditions exhibit most
of their growth in the first 25 percent of their life prior
to hail. Growth is still apparent in the last 25 percent
time interval before hail for the cold and stationary
frontal cases. The differences in the amount of growth
between figures 2 and 3 result from the different means of
expressing time (real time versus percentage time).
Several comparisons between the 50 hail-echo tops and
the 50 no-hail-echo tops, all of which formed in range,
were made. Figure 4 is a real time average height graph
depicting curves for hail and no-hail echoes as stratified
by synoptic weather conditions. The average hail-echo
tops were higher than those of the no-hail echoes in all
but the earliest 20 min of stationary frontal echoes.
The no-hail echoes exhibited very little growth after the
1 l-20-min time interval, whereas the hail echoes almost
always exhibited some increase after this interval.
Table 5 shows probabilities for different frequencies of
taller echoes that will produce hail on any given hail day.
Values indicate the possible confidence in predicting that
tall echoes will be hailstorms. These probabilities are
based on a comparison of the heights of the 50 hail echoes
with those of the 50 no-hail echos at formation, at average
hailtime, and at dissipation. The taller half of the echoes
on each day were used in this analysis. The probabilities
at formation time show that on 38 percent of the days
81 percent or more of the taller echoes will become
hailstorms and that on 54 percent of the days more than
60 percent will become hailstorms. On 84 percent of the
hail days, more than 60 percent of the taller half of the
echoes a t hail time and a t dissipation will be hailstorms.
FIGURE 3.-Height curves of hail echoes for three synoptic weather
categories as determined for the indicated percent of time from
formation to the time of hail.
Since prediction at formation time is the most useful
operational knowledge, it is important to realize that
on 77 percent of the days more than half of the taller
echoes at formation became hailstorms, and on 54 percent
of the hail days more than 65 percent of the taller echoes
became hailstorms.
Figure 5 is a plot of the echo heights against percentage
of total echo life for'all the hail and no-hail echoes that
formed and dissipated in range. The hail echoes were
higher than the no-hail echoes, but the shapes of the curves
are quite similar with a constant difference in height of
about 2,500 f t . This suggests similar processes in cloud
echo evolution, but more vigorous convection throughout
the life of a hailstorm. In a study of heights of 33 echoes
at hail time, as depicted on an RHI of a 3-cm TPS-10
radar, Changnon (1969) calculated an average height of
29,600 ft. The average height at hail time for the 35 hail
echoes that formed and dissipated in range was 27,000 ft.
10. COMPARISONS OF HAIL AND NO-HAIL ECHOES
The comparisons of characteristics of hail echoes and
no-hail echoes are summarized in table 6. In general, the 50
hail echoes that formed in range were used for the compar-
ison. As can be seen, there is very little difference in the
values for echo time, direction, speed, and reflectivity at
echo formation. There was some difference in the average
reflectivities at the average hail time (44 min) after echo
formation. For the echoes that formed and dissipated in
range, the average heights of the hail echoes were higher
than those of the no-hail echoes at all times.
11. SYNOPTIC WEATHER CONDITIONS
The durations of echoes between formation and first
hail were determined for three primary synoptic cate-
gories. Cold frontal storms tend to have longer prehail
durations than do echoes with the other categories with
50 percent having long (2 5 0 min) durations. Echoes
formed under stationary frontal conditions showed about
equal preference for short (32 percent) , moderate (32
percent), and long (36 percent) durations. Echoes with
3 50 MONTHLY
15
30
AVERAGE
--------
30
25
COLO FRONT
AVERAGE
----
STATIONARY FRONT
I I I I I 
I I I I I
$ w 2 5 L
= 20
LOW
WEATHER REVIEW voo. 98, No. 5
NO-HAIL ECHOES 
; 20
2
I
Y
I
7 1 5 1 0 25 50 75 100
FORMAT1 ON DISSIPATION
CUMULATIVE PERCENT OF TIME
FIGURE 5.-Height curves for all hail and nehail echoes as de-
termined for the indicated percent of time from formation to
dissipation in range.
TABLE 6.-Comparison of charactwistics of hail echoes with those of no-hail echoes*
FIGURE 4.-Height curves of hail and no-hail echoes for three
major synoptic weather categories for time intervals after for-
mation.
TABLE 5.-Daily probabilities that the tallest half of the echoes at different stages will produce hail
Probability, percent, at dmerent stages
Formation ., Hail time* Dissipation Percent of tallest half of the echoes
*The heights used for analyzing no-hail echoes were those at the time corresponding
to the average time of hail (44 min after formation), as based on all 60 hail echoes.
Lows produced hail more quickly after echo formation
with 50 percent producing hail in less than 29 min.
The analysis of speed with each synoptic category
showed in general that the cold frontal echoes moved
with moderate (20-29 kt) to fast (2 3 0 kt) speeds with
90 percent moving faster than 19 kt. More than 40 per-
cent moved with fast (2 3 0 kt) speeds. Stationary frontal
echoes moved with slow (I 19 kt) to moderate (20-29
kt) speeds. Forty-seven percent moved with slow speeds,
and 43 percent moved with moderate speeds. Low echoes
moved with moderate speeds. More than 70 percent
moved with moderate speeds. An analysis of echo speeds
before, during, and after hail for each synoptic category
showed no signifhant differences against time.
The directions of echo motion (toward which the 103
echoes were moving) were grouped by synoptic categories
(fig. 6). Stationary frontal echoes (fig. sa) had a marked
Echo characteristics Hail No-hail
Preferred time of occurrence, CST _____ __ ____ __ ______. _____. ___ __.
Average direction of movement, degrees- ____. _____. . ____
~
._ ____ Average speed, kt - - - __ - - - .- -. ___
~
- __ -
~ ~
__
~
- - ~.
~
__ .-. - - - -
~
- - - - -
~
Average reflectivity at formation, 1111116 mJ. _____ __. . ____ __ __ ___ Average reflectivity at hail time,+ 1111110 m-s_-. . __ ___. __ _.__ __ -.
Average height at formation, It ______..____ ____ ____..
~
____._____
Average height at hail time,t ft ____ ___ ___. ___ ___. __ __. . __ __ ~. ~.-
Average height at dissipation, ft. __ __ ____ __ - ___ __ __ __ ____ __. ____
1200-1800
81.9
23.8
6.1XlDa
7.3XlO1
25,600
27, OD0
21,800
1200-1800
85.9
24.2
5.5XlG
1.6XlO1
23, OOO
24,700
18,500
All values are based on the 50 no-hail echoes and on the 50 hail echoes that formed in
range, except the height values which are bnsed on the 35 hail echoes that formed and
dissipated in range.
tAvexage hail time was 44 min after formation.
tendency to move in a northeast direction, and those
with Lows frequently moved t o the east-southeast and
southeast. The cold frontal echoes showed a preference
for northeast or southeast motions. When the directions
of all 103 echoes were grouped, preferences for northeast
and east-southeast were indicated (fig. 6d).
An analysis by synoptic category was also done for the
echo turning in the period prior to first hail, according to
left turn, right turn, or no turn. This analysis revealed
no marked preference Rmong the three turn options for
the echoes with cold and stationary fronts (figs. 7a and
7b). The echoes with Lows had more of a tendency to
turn to the left, or to not turn, than to turn to the right
(fig. 7c). This indicates considerable variability in steering
level winds with each category.
Also shown in figures 7a, 7b, and 7c are the average
degrees of turn to the right or left for each synoptic
situation. The cold frontal echoes which turn tend to have
a 50 percent greater turn to the right than to the left,
whereas the echoes with other synoptic categories average
about the same degrees of turn to the right as to the left.
May 1970 Neil G. Towery and Stanley A. Changnon, Jr. 351
a. Stationary front b. Lon
7 4 3 20% 10% I 0 10%
m m
B
9 9
C . Cold front d . All cases
FIGURE 6.-Percent of occurrence of hail-echo motion (toward which
the echoes were moving) for each synoptic weather category and
for all cases.
When the times of day for the beginning times of the
103 echo tracks were grouped by synoptic situations, the
cold frontal cases were found to occur largely in the after-
noon and evening. Afternoon tendency for beginnings was
very predominant for the stationary frontal echoes. The
echoes with low conditions had a decided preference for
beginning in the morning and afternoon. Diurnal heating
was obviously an important factor in hailstorm occurrence
in all three classes.
19. ECHO MODELS
Analyses of the echo characteristics,: when sorted and
then grouped for the three synoptic weather categories,
revealed distinctly different and reasonable models for each
(table 7). These synoptic models, or typical hail echoes,
provide some information that can be used as guidance in
making operational decisions concerning potential hail-
producing echoes.
The hail-echo model for cold fronts is faster moving, as
would be expected from the normal upper level steering
winds with cold fronts, than are the other echo models.
380-965 %70---3
1 URN
FIGURE 7.-Percent of occurrence of hail echoes turning and not
turning and average degrees of turns for each synotic weather
category.
The cold frontal model also is the longest lived and highest
storm (entire duration), and has relatively high reflectivi-
ties. The considerable instability associated with cold
fronts indicates these findings are reasonable. The high
reflectivity values also may relate to the fact that hail from
cold frontal storms is relatively long lasting and that cold
frontal hailstorms usually are associated with relatively
heavy rainfall (Changnon 1969).
The stationary frontal model of hail echoes indicates
a right turn prior to the development of hail. Newton
(1963) has indicated that severe storms embedded in warm
moist air masses (which is the case for this condition) tend
to obtain their indraft air at low levels along their south-
east flank. Thus, new growth develops along the right
flank which results in an apparent right turn in such 8.n
environment. The preference for afternoon echo develop-
ment in the stationary frontal conditions further reflects
the importance of low-level local heating on the develop-
ment of hail echoes under this condition. The tendency for
a left turn by hail echoes with cold fronts suggests that
the heavier precipitation in these storms effectively blocks
the primary upward inflow for the storm (Phillips 1969).
The primary flow circles inward on the left flank, which
results in displacement of the updraft to the left and
growth on the left flank in many cold frontal storms.
The stationary frontal hail-echo model (table 7) is
shown by its reflectivity and height values to be a strong
vigorous storm. This is not unexpected. Changnon (1960b)
352 MONTHLY WEATHER REVIEW Vol. 98, No. 5
TABLE 'I.-Summary of echo characteristics for each synoptic
classification
Stationary Echo characteristies Coldfront front Low
Average speed (kt) ____.._._________._.-.-.-----.---
Average duration prior to hail (min)---. . -. -. . __ __.
Average total duration (min). ___. .- - -.. ___ __. . .. -.-
Preferred direction of motion..- ___ __. . __ __. ___ .- .--
Preferred direction of turn ___.._.______.___.__.-----
Average number of degrees turn (when turning) in
preferred direction _______________
~
_____.__________
Preferred time of day, C~T--- - - _.__. - _________.__ ___ Average reflectivity at formation (m a m-3) __._. __ -
Average reflectivity at hail time (m o r n -3 ) ________ Average reflectivity at dissipation (mmam-3) .-.-...
Average top height for echo duration (lol ft) _____.._
Average top height at hail time (lol ft) _____ -. -. __. -
30
59
90
NE
Left
12
5.6X101
2.6X105
3.6X101
38
37
12W2400 1
21 25
49 32
83 75
NE ESE
Right Noturn
17 0
L200-18M m1800
5.1X101 4.6XIOa
4.6XlOS 5.2X104
9. lXlW 2.6Xlol
36 19
38 20
indicated that a large number of damaging Illinois hail-
storms were produced under stationary frontal conditions,
and a recent study of hailstreaks (Changnon 1969) shows
that the volume of hail per unit area was quite large from
hailstorms occurring during stationary frontal conditions.
The typical hailstorm produced by low conditions is
the weakest and shortest lived of the three synoptic
weather models. These storms exhibit a capability of
producing hail fairly quickly after echo formation, but
in turn the echo life is considerably shorter than those of
the other models.
In general, the values in table 7, which are considered
t o be models of Illinois hail echoes, appear t o be reasonable
because they are in agreement with prior findings on
surface hail, instability with severe weather, and the
mechanics of hailstorm development.
13. SUMMARY AND CONCLUSIONS
This study has considered various parameters associated
with hail-producing echoes in Illinois. Those parameters
included echo location, duration, direction of motion,
speed, time of day, associated synoptic weather condi-
tions, and their relationships with each other. I n addition,
analyses were done on echo reflectivities and heights.
The echo location analysis indicated that the echoes
have a slight tendency to form in the area northwest of
the radar site. Dissipation location of the hail echoes
shows a preference for the north sector, a condition related
t o the preferred area of formation. The sample was too
small to ascertain any small (1,000 sq mi) areas of echo
development, but the findings concerning greater hail-
echo frequency in the northwest and north sectors agree
with climatological findings on warm-season-average hail-
day frequencies (Changnon 1963) which show a maximum
in these areas of central Illinois.
The average height of the hail-echo tops revealed a
5,000-ft increase during the 10-15 min prior to f i s t hail
(fig. 2) which was the most rapid growth on the average
echo height profile. This indicates that a sustained up-
draft surge was related to the hail production, and this
could fit the hailstorm model proposed by Gaviola and
Fuertes (1947) and subsequently elaborated on by Ludlam
(1958), 8s well as the Bates model (1965).
The most striking finding from this hail-echo study was
the great variability. Hail-producing echoes had maximum
tops ranging anywhere between 9,000 and 54,000 ft at
the time of hail, lifetimes from 30-197 min, average
speeds from 5-50 kt, reflectivities at hail time from IO2
to lo8 mm6 mW3, and were produced by all types of synoptic
weather classifications that produce summer precipitation
in Illinois. Consequently, the establishment of a single
model of a hail-producing echo would be difficult, and
any such model would be relatively meaningless. However,
three synoptic models were developed as discussed in the
previous section.
Comparison of the characteristics of the hail-producing
echoes with those of no-hail echoes on the same days to
discern forecasting guides revealed great similiarity in all
aspects except echo height. Throughout the echo duration
for each synoptic category, the hail-echo top had an
average height that was between 2,000 and 4,000 ft higher
than that of the no-hail echo. The similarity in the shapes
of the time-height curves of the average hail echo and the
no-hail echo indicates a similar evolution of growth and
dissipation of convection. However, the continuously
greater height of the average hail echo indicates 1)
stronger early convection prior to echo development,
and 2) sustenance of greater convection throughout its
duration. Thus, as has been shown by Douglas (1963)
for Alberta hailstorms, the probability of hail in an
Illinois storm is tied to the degree of vertical development
of a storm.
Two-thirds of the echoes turned to the right or left
prior to hail production. However, there was no marked
preference for right or left turns.
Echo speed at time of hail was not markedly different
from that prior to and after hail. Thus, changes in echo
speed could not be used to indicate hail-producing echoes.
The average echo-top heights at hail time shown in
table 7 for the two frontal models agree remarkably well
with the average maximum heights for frontal thunder-
storms in Ohio (Byers and Braham 1949). The average
total durations of the hail echoes, 75-90 min (table 7),
were 15-30 min longer than those found for thunderstorm
echoes in Ohio (Byers and Braham 1949).
ACKNOWLEDGMENTS
This research was supported by funds from the Atmospheric
Sciences Section of the National Science Foundation, NSF GA-
4618, and the State of Illinois. The advice and suggestions of
Donald W. Staggs, Floyd A. Huff, and J. Loreena Ivens of the
Survey staff are appreciated.
May 1910 Neil G. Towery and Stanley A. Changnon, j r . 353
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[Received September 22, 1969; revised November 2.4, 19691