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The TORRO Hailstorm Intensity Scale
The TORRO Hailstorm Intensity Scale was introduced in 1986 by Jonathan Webb from Oxford,
Oxfordshire (U.K.) to categorize damage caused by hailstorms.
The potential damage that a hailstorm can cause
is generally proportional to the crystal size and
fall speed. In addition to the size and speed of
fall, other components to be considered are the
hardness, the shape and orientation of the fall
trajectory.
The intensity of a hailstorm can be more easily
determined if it takes place on solid surfaces of
objects that have the ability to maintain obvious
damage or when it occurs on a constructed area.
The intensity of a hailstorm is determined in
reference to the most damage that is caused.
When a hailstorm occurs in open country, where the damage cannot be measured, and the intensity
of the phenomenon is related to the size of the hailstone rather than the damage that it could have
potentially caused. When the damage is not evident, however, it is assigned the lowest category.
The same criteria are used in cases where damage cannot be quantified: for example, a hailstorm
with crystals the size of eggs can potentially cause damage in the H6-H8 range (see table).
If the damage cannot be quantified, the hail is relegated to the first lower limit, i.e. H5.
In conclusion, it is possible to say that there is a close relationship between crystal size and damage.
Categories were created which include a multiplicity of damage to better classify events.
It is in fact possible that very large hailstones cause less damage because they are within strong
crosscurrents rather than other smaller hailstones which are however inside tornadoes.
The table that follows refers specifically to damage caused and is categorized in a scale (the Torro
Hailstorm Intensity Scale). The last column is the “code-size-range” that will serve us in the second
table in identifying the phenomenon based on size.
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Using the Hailstorm Intensity Scale
The intensity of a hailfall (left column) is assigned according to the maximum damage observed in
the first table below, providing that the hailstorm occurs over an area where there are many varied
objects with differing capacities to hold damaging traces (such as a built-up area). The intensity
of the overall storm is rated according to the highest hailfall intensity (cf tornadoes and tornado
intensities).
The hailstone size code range (right-hand column) gives an indicator of the likely stone diameter
for each intensity. The second table below specifies the actual diameter range relating to each size
code.
TORRO
INTENSITY
DESCRIPTION OF DAMAGE
(FOR GUIDANCE ONLY)
H0
True hail of pea size but no damage.
H1
Leaves holed; flower petals cut.
1-3
H2
Leaves stripped from trees and plants; vegetables, fruit and crops
bruised and scarred; vegetable leaves shredded.
1-4
H3
A few panes in glasshouses, cloches and / or skylights broken; wood
(fences) scored; paint scraped off window ledges; caravan bodywork
dented; perspex roofing holed; canvas (e.g. tents) torn; stems of crops
severed and seeds threshed; fruit sliced / split.
2-5
H4
Some house windows and / or vehicle windscreens broken / cracked;
glasshouses extensively damaged; some felt roofs pierced; paint
scraped off walls and vehicles; some thin car bodywork visibly dented;
small branches broken from trees; unprotected birds and poultry killed;
firm ground pitted.
>3-6
H5
Some roof slates and pottery-type tiles broken; many windows smashed;
plate-glass roofs and reinforced glass windows broken; bodywork of
most exposed cars visibly pitted; bodywork of light aircraft pitted; risk
of serious or fatal injuries to small animals; strips of bark torn from trees;
woodwork pitted and splintered; large branches cut from trees.
4-7
H6
Many roof slates and tiles (excepting concrete) broken; shingle and thatch
roofs breached; corrugated iron and some sheet metal roofs scored and
a few holed; brick walls slightly pitted; wooden window frames broken
away.
5-8
H7
Slated, shingle and many tiled roofs shattered, exposing rafters; metal
roofing punctured; brick and stone walls pitted; metal window frames
broken away; bodywork of cars and light aircraft seriously / irreparably
damaged.
6-9
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RANGE OF
HAILSTONE
SIZE CODE
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TORRO
INTENSITY
DESCRIPTION OF DAMAGE
(FOR GUIDANCE ONLY)
RANGE OF
HAILSTONE
SIZE CODE
H8
Concrete roof tiles cracked; sheet metal, slated, shingle, and other tiled
roofs destroyed. Pavements pitted; bodywork of commercial aircraft
severely damaged; small tree trunks split apart; risk of serious injury to
persons caught in the open.
7-10
H9
Concrete walls pitted; concrete roof tiles widely broken; walls of wooden
houses completely holed; large tree trunks cut down; risk of fatal injury to
persons caught in the open.
8-10
H10
Wooden houses destroyed; brick-built houses very severely damaged;
risk of fatal injury to unprotected persons.
9-10
A hailfall across open land may not have the opportunity to cause damage, while damageable
objects may not be capable of holding the maximum amount of damage that a stone of a given
size can inflict.
In such circumstances, the diameters of the largest stones are measured to obtain a mean figure.
The corresponding diameter range is identified in the table below (second column).
The third column assists in diameter range identification, if descriptive terms are the only indicator
of stone size.
The fourth column gives the corresponding range of likely intensities. With no damage to rate a
hailfall / hailstorm with, the lowest intensity code is always assigned.
SIZE CODE
DIAMETER
1
5 - 10 mm
Pea.
H0-2
2
11 - 15 mm
Mothball, bean, hazelnut.
H0-3
3
16 - 20 mm
Marble, cherry, small grape.
H1-4
4
21 - 30 mm
Large marble, large grape, walnut.
H2-5
5
31 - 45 mm
Chestnut, pigeon’s egg, golf-ball, table-tennis ball, squash ball.
H3-6
6
46 - 60 mm
Hen’s egg, small peach, small apple, billiard ball.
H4-7
7
61 - 80 mm
Large peach, large apple, goose egg, small / medium orange,
tennis ball, cricket ball, baseball.
H5-8
8
81 - 100 mm
Large orange, grapefruit, softball.
H6-9
9
101 - 125 mm
Melon.
H7-10
10
over 125 mm
Coconut, etc.
H8-10
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Some exact diameters, with size codes, are:
Five pence (new) coin 17 mm (3); one penny coin 20 mm (3); twenty pence coin ~21 mm (4); one
pound coin 22 mm (4); ten pence (new) coin 23 mm (4); two pence coin 26 mm (4); fifty pence coin
~31 mm (5); table tennis ball 38 mm (5); golf ball 41 mm (5); tennis ball 68 mm (7); cricket ball 73
mm (7).
Imperial coins: six pence 19 mm (3); half crown 32 mm (5).
Via Lancia 120 - 10141 Torino - Italy
Tel +39 011 3358041 Mob +39 331 6343682
aral-srl.com
Fax +39 011 377833
P.iva 04540090018
info@