During storm season, there is increasing talk about severe thunderstorms. What does “severe” in severe thunderstorms mean? When does a thunderstorm become severe? What criteria does it have to meet to be considered severe?
Severe thunderstorms comprise a relatively small percentage of all thunderstorms. The majority of thunderstorms are not severe. For a thunderstorm to become severe, the environmental factors need to be right: the presence of high to extreme instability, strong wind shear, cold and dry mid-levels, etc.
Thunderstorms’ primary damage sources include hail, rain, wind, and lightning.
The following are severe weather criteria defined by the European Severe Storms Laboratory. They differ somewhat from the requirements in the US as defined by the National Weather Service.
LARGE HAIL
Hail comes in different shapes and forms – single hailstones and agglomerates. Single hailstones form an onion-like internal formed during the growth of the hailstone. Agglomerate hailstones are composed of numerous smaller hailstones.
Regardless of the type, a thunderstorm (hailstorm) is considered severe when the maximum hailstone size exceeds 2.0 cm. Hail 2.0 cm is considered marginally large, 2.0 – 5.0 cm is considered large, and at >5.0 cm, it is considered very large.
Left: Huge hailstones near Krsko, Slovenia on July 13th, 2023, photo by Tina Novsak; Right: Giant hail in Tripoli, Libya on October 27th, 2020, photo by Saad Aldeen Hmouda
Another definition of a severe hailstorm has been introduced, considering significant hail accumulations. Regardless of hailstone size, a hailstorm is considered severe if it produces flat ground accumulations 2.0 cm thick. Accumulations by local water streams (storm surface runoff) do not count.
WIND
Wind includes straight-line winds and wind vortices. Straight-line winds are produced in downbursts in which mid-level air rapidly cools and plunges towards the ground, extending outwards. Straight-line winds are also made in bow echoes, thunderstorms characterized by strong straight-line winds at the surface.
Vortex phenomena usually include mesocyclonic and non-mesocyclonic tornadoes. Regardless of the type of wind and its cause, the severe limit is 90 km/h = 25 m/s.
Severe downburst and tornado damage around Črnivec and Gozd, central Slovenia, July 13, 2008. Image: Space.si
RAINFALL
Rain produces extreme impact, including making essential streets impassable, disrupting rail, tram, or subway transport, flooding multiple structures or their basements, causing significant damage to structures or vegetation, and requiring emergency services to act numerous times.
Flooding of Unica river at Planinsko polje, Slovenia on November 19th, 2019. Photo by Marko Korosec
It does not include flooding along rivers and flooding due to or in part due to thawing. The limit is 25 mm of rainfall in 30 minutes and 35 mm in 60 minutes.
LIGHTNING:
No lightning rate is set as the severe limit; instead, the effect of a lightning storm is considered – a lightning storm is considered severe if lightning has caused significant damage to aircraft, vehicles, ships, or structures or has injured or killed people or animals.
Cloud-to-ground lightning strike by Marko Korošec.
More information on severe weather criteria in Europe is available in ESWD Severe Weather Reporting Criteria.
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