Feb 2007


Facts about Hailstorms:

  • Hail develops during most mid-latitude thunderstorms, often melting before reaching the ground.
  • Hail forms in the updraft of a thunderstorm with stronger storms producing larger hail. The updrafts pull the precipitation up into the cloud, adding layer upon layer of moisture.
  • Hail is much more common along mountain ranges because mountains force horizontal winds upwards, thereby intensifying the updrafts within thunderstorms and making hail more likely.
  • Cut in half, a hailstone has concentric rings like an onion, which reveal the number of times it traveled to the top of the storm before falling to Earth.
  • Typically, hailstones can range from pea-sized (0.25 in.) to golf ball sized (1.75 in.) to softball-sized (4.50 in.)
  • On June 22, 2003, a hailstone recovered in Aurora, NE, had a diameter of 7 in. (17.8 cm) and a circumference of 18.75 in. (47.6 cm). This hailstone was larger than the previous record large hailstone that fell in Coffeyville, KS in 1970 (5.7 in. (14.5 cm) diameter and 17.5 in. (44.5 cm) circumference). However, an accurate weight could not be determined for the Aurora hailstone; so the Coffeyville hailstone remains the heaviest hailstone weighed and verified in the United States at 1.67 pounds (0.76 kg).
  • Severe hail is officially defined as being 0.75 in. or more in diameter. But even most hail which is technically “severe” measures an inch or less across – too small to cause personal injury or serious property damage, except to crops (which can be destroyed even by very small, wind-driven hail).
  • Hailstones can fall at speeds up to 120 mph (53 m/s)
  • Hail causes $1 billion in damages to crops, roofs, automobiles and other outdoor objects and property each year.
  • Extremely severe hailstones can destroy cars, ruin roofs, break windows, kill animals and seriously hurt or kill humans. The costliest thunderstorms in history have been supercells producing very large hail over metropolitan areas – such as Fort Worth, TX in 1995 and Sydney, Australia in 1999.
  • Costliest United States hailstorm: Fort Worth, Texas, May 5, 1995. Total damage was $2 billion. The next costliest hailstorm occurred in the area of St. Louis, MO with a total damage of $1.9 billion.

While hailstorms are widespread, severe hailstorms that produce hailstones large enough to cause damage are known to fall in certain geographic regions of the United States on a more frequent basis. Plants and facilities within these areas known to experience frequent severe hailstorms should take certain precautions in the design/construction/maintenance of buildings, roofs, roof-mounted equipment and other outside structures/equipment.

Hail damage to built-up roof (BUR) coverings can be greatly reduced by providing slag or gravel surfacing adhered with a flood coat of hot asphalt. If a smooth surface roof is necessary, provide three to four plies of glass fiber felts bonded to each other with a full mop of asphalt to provide the best hail resistance (but not as good as a BUR with slag/gravel surfacing).

Besides slag/gravel surfaced BUR covers, single-ply roof coverings that have been specifically tested and approved (by FM Approvals) for hail impact can also be used to mitigate or prevent hail damage to roof coverings. The insulation used under these approved single-ply coverings should be approved with the particular membrane being used.

New outdoor equipment such as duct work, lightweight sheet metal housings, air conditioning fins, HVAC equipment, ventilators, dust collectors, skylights, etc. should be designed to resist, without permanent deformation, the impact energy of a 2 in. (51 mm) diameter hailstone. Due to unfavorable loss experience, skylights deserve special attention.

When outdoor equipment has not been designed to resist hail impact, or does not have sufficient inherent strength to resist severe hailstorms, hail screens or guards may be placed over the equipment. This particularly applies to skylights. The protectors may be deflected by impact, but the energy of the stones will be considerably dissipated and damage to the equipment/skylight will be prevented.

Larger hailstones can easily break blisters on existing BUR coverings, whether they have slag/gravel surfacing or not. This will destroy the water tightness of the roof covering and lead to leaks and possible damage to the building and contents. Blisters in roof coverings should be cut, patched and resurfaced as necessary.

Hailstorms are not confined to the United States, they happen worldwide. One of the most notorious regions for large hail is northern India and Bangladesh, which have reported more hail-related deaths than anywhere else in the world. In fact, some of the largest hailstones ever measured have fallen in these areas.

Mainland China is also notorious for killer and damaging hailstorms. On July 22, 2002, hailstones reportedly the size of eggs killed 25 people and injured over 200 in China’s central Henan province. Livestock were killed, buildings collapsed and hundreds of automobiles were damaged by the unusually large hailstones.

In 2001, livestock, houses and farmland in seven villages were destroyed by hailstones that were reportedly the size of footballs and weighing up to 2 kg in the northern Nigerian city of Kano.

Also in 2001, large hailstones the size of cricket balls fell during widespread storms in Australia causing much damage. The costliest foreign hailstorm occurred in Sydney, Australia on April 14, 1999. Total damage was $1.6 billion. 20,000 properties and 40,000 vehicles were damaged during the storm with more than 25 aircraft damaged at Sydney Airport.

In 1986, the Tornado and Storm Research Organization (TORRO) published a Hailstorm Intensity Scale that it had developed in order to characterize the more than 2,500 hailstorms known to have occurred in the British Isles since the first documented event of 1141 AD. The most intense of these storms reached intensity H8 on this intensity scale which extends from H0 to H10. In the year 2004, 16 hailstorms reached intensity H3 or H4 – the severe category – capable of damaging glass and other light structures and (at H4) vehicle bodywork.

Risk Logic Inc. will evaluate your facility for its exposure to and protection against damaging hailstorms during their comprehensive property loss prevention survey. If you would like to schedule such a survey, please contact Risk Logic.

1. FM Global Loss Prevention Data Sheet 1-34, “Hail Damage”
2. Federal Emergency Management Agency
3. BBC News
4. Tornado & Storm Research Organization, Department of Geography, Oxford Brookes University, Oxford, OX3 0BP, United Kingdom

Risk Logic, Inc. can be of assistance regarding fire protection Codes and Regulations. We can ensure that the above codes as well as NFPA and FM Global standards are being followed in an effort to avoid a major property loss.