Ranked scale

The TORRO Hailstorm Intensity Scale

Hail graded by what it breaks, not just by how big it is.

Origin

Developed by the Tornado and Storm Research Organisation alongside its tornado scale, and used to assess hailstorms in Britain and across Europe.

The catch

Stone size alone tells you surprisingly little. A large, soft hailstone falling through still air can do less damage than a smaller, harder one driven by strong wind, so the scale grades a storm by size, hardness, number of stones and wind together. Size is given only as a typical guide.

H0 — Hard hailPea, about 5 mm (0.2 in)

No damage. The commonest hail in Britain by a wide margin.

H1 — Potentially damagingPea to marble, 5–15 mm (0.2–0.6 in)

Slight general damage to plants and crops.

H2 — SignificantMarble to grape, 10–20 mm (0.4–0.8 in)

Significant damage to fruit, crops and garden vegetation.

H3 — SevereWalnut, 20–30 mm (0.8–1.2 in)

Severe damage to fruit and crops. Glass and plastic structures broken, paint and wood scored.

H4 — SeverePigeon’s egg, 25–40 mm (1.0–1.6 in)

Widespread glass damage and dented vehicle bodywork.

H5 — DestructiveGolf ball, 30–50 mm (1.2–2.0 in)

Wholesale destruction of glass, damage to tiled roofs, and a real risk of injury to anyone outdoors.

H6 — DestructiveHen’s egg, 40–60 mm (1.6–2.4 in)

Bodywork of grounded aircraft dented, brick walls pitted.

H7 — DestructiveTennis ball, 50–75 mm (2.0–3.0 in)

Severe roof damage and a risk of serious injury. Several British storms have reached this level, including Horsham in 1958.

H8 — DestructiveLarge orange, 60–90 mm (2.4–3.5 in)

Severe damage to aircraft bodywork. The most intense level ever recorded in Britain, reached only once.

H9 — Super hailstormGrapefruit, 75–100 mm (3.0–3.9 in)

Extensive structural damage. Risk of severe or fatal injury to anyone caught in the open.

H10 — Super hailstormMelon, over 100 mm (over 3.9 in)

Extensive structural damage. Risk of severe or fatal injury to anyone caught in the open.

Most British hail is H0: small hard pellets that rattle off a windscreen and do nothing at all. The only British hailstorm ever rated H8 struck Hertfordshire, between Hitchin and Great Offley, in May 1697, tearing up the ground and splitting oak trees. The heaviest hailstone officially recorded in Britain fell on Horsham during the H7 storm of 5 September 1958, which tracked some 150 km from West Wittering in Sussex to Maldon in Essex; TORRO gives its weight as 190 g, though some sources quote around 140 g. The longest-tracked British hailstorm, an H6 on 22 September 1935, ran 335 km from Newport in Gwent to Mundesley in Norfolk.

Common questions

What is the biggest hailstone ever recorded in the UK?

The Horsham hailstone of 5 September 1958, which TORRO records at 190 g. It fell during an H7 storm.

What size hail causes damage?

From H1 (5–15 mm) hail starts to damage plants and crops. Glass and car bodywork are at risk from around H3–H4, roughly 20–40 mm or walnut-sized and up.

How often does Britain get damaging hail?

Hail of H3 or above occurs somewhere in Britain most years, usually in summer and most often in central and eastern England. H5 and above is rare.