The TORRO Hailstorm Intensity Scale


The scale extends from H0 to H10 with its increments of intensity or damage potential related to hail size (distribution and maximum), texture, numbers, fall speed, speed of storm translation, and strength of the accompanying wind. The characteristic damage associated with each increment in Britain is listed in the table but may need to be modified for other countries to reflect differences in building materials and types; e.g. whether roofing tiles are predominantly slate, shingle or concrete.
Large hailLarge hail measuring 4.9 cm in diameter (1.92 inches)
(Credit: Paul Knightley)

An indication of equivalent hail kinetic energy ranges (in joules per square metre) has now been added to the first six increments on the scale, and this may be derived from radar reflectivities or from hail pads. The International Hailstorm Intensity Scale recognizes that hail size alone is insufficient to accurately categorise the intensity and damage potential of a hailstorm, especially towards the lower end of the scale. For example, without additional information, an event in which hail of up to walnut size is reported (hail size code 3: hail diameter of 21-30 mm) would be graded as a hailstorm with a minimum intensity of H2-3. Additional information, such as the ground wind speed or the nature of the damage the hail caused, would help to clarify the intensity of the event. For example, a fall of walnut-sized hail with little or no wind may scar fruit and sever the stems of crops but would not break vertical glass and so would be ranked H2-3. However, if accompanied by strong winds, the same hail may smash many windows in a house and dent the bodywork of a car, and so be graded an intensity as high as H5.

However, evidence indicates that maximum hailstone size is the most important parameter relating to structural damage, especially towards the more severe end of the scale. It must be noted that hailstone shapes are also an important feature, especially as the "effective" diameter of non-spheroidal specimens should ideally be an average of the co-ordinates. Spiked or jagged hail can also increase some aspects of damage.

The H Scale

The table below highlights the The TORRO Hailstorm Intensity Scale (H0 to H10) in relation to typical damage and hail size codes...

Scale Intensity category Typical hail diameter (mm)* Probable kinetic energy J m-2 Typical damage impacts
H0 Hard hail 5 0-20 No damage
H1 Potentially damaging 5-15 >20 Slight general damage to plants, crops
H2 Significant 10-20 >100 Significant damage to fruit, crops, vegetation
H3 Severe 20-30 >300 Severe damage to fruit and crops, damage to glass and plastic structures, paint and wood scored
H4 Severe 25-40 >500 Widespread glass damage, vehicle bodywork damage
H5 Destructive 30-50 >800 Wholesale destruction of glass, damage to tiled roofs, significant risk of injuries
H6 Destructive 40-60   Bodywork of grounded aircraft dented, brick walls pitted
H7 Destructive 50-75   Severe roof damage, risk of serious injuries
H8 Destructive 60-90   (Severest recorded in the British Isles) Severe damage to aircraft bodywork
H9 Super Hailstorms 75-100   Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open
H10 Super Hailstorms >100   Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open

* Approximate range (typical maximum size in bold), since other factors (e.g. number and density of hailstones, hail fall speed and surface wind speeds) affect severity.

Hail size and diameter

The table below highlights the hail size and diameter in relation to TORRO Hailstorm Intensity Scale...

Size code Maximum diameter (mm) Description
0 5-9 Pea
1 10-15 Mothball
2 16-20 Marble, grape
3 21-30 Walnut
4 31-40 Pigeon's egg > squash ball
5 41-50 Golf ball > Pullet's egg
6 51-60 Hen's egg
7 61-75 Tennis ball > cricket ball
8 76-90 Large orange > soft ball
9 91-100 Grapefruit
10 >100 Melon

The Size Code is the maximum reported size code accepted as consistent with other reports and evidence.