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Thunderstorms

A thunderstorm is a complex meteorological event, characterised by the presence of electrical discharges (lightning) followed by thunder. The conditions needed for a thunderstorm to develop are a sufficient level of humidity and the rapid upward movement of air, e.g. as a result of being warmed. Inside a storm cloud (cumulonimbus) there are often strong updrafts and downdrafts, which cause turbulence. Hazardous weather events that can occur in a thunderstorm include lightning, heavy precipitation (downpour of rain, hail, graupel/snow pellets, and, in winter, snow) as well as wind gusts that can sometimes reach up to hurricane strength.

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Hazard level 5 (very severe hazard)

Thunderstorm warnings are only issued for categories 3 and 4. In spite of the most up-to-date tools and meth-ods, it is not currently possible to predict or give several hours’ warning of the strength, timing and location of severe thunderstorms. Three of the categories are therefore of little significance.

Hazard category 4 (severe hazard)
Possible impacts
  • Sudden flash flooding of streams
  • Breaking of bigger branches, toppling of trees, possibility of landslides on steep slopes
  • Severe damage caused by hail
  • Lightning strikes
  • Failure of drainage and sewer systems
  • Flooding of underpasses, underground garages and cellars
  • Disruption of and danger to road, rail, water and air traffic (e.g. due to sudden, very strong gusts of wind, flooding, toppled trees)
Description of measurement and prognostic values Very severe thunderstorm (flash storm, localised; warning time 0 to 1 hour): >120 km/h, >4 cm hailstones, >50 mm/h

Hazard category 3 (significant hazard)
Possible impacts
  • Sudden flash flooding of streams
  • Breaking of branches, toppling of individual trees, possibility of landslides on steep slopes
  • Damage caused by hail
  • Lightning strikes
  • Failure of drainage and sewer systems
  • Flooding of underpasses, underground garages and cellars
  • Disruption to road, rail, water and air traffic (e.g. due to sudden, very strong gusts of wind, flooding, toppled trees)
Description of measurement and prognostic values Severe thunderstorm (flash thunderstorm; localised, warning time 0 to 1 hour): 90-120 km/h, 2-4 cm hailstones, 30-50 mm/h

Hazard category 2 (moderate hazard)

A thunderstorm event that is usual for the time of year (for which, therefore, no special warnings are issued).

Recommendations for action

What to do during a thunderstorm

Notes

When widespread severe thunderstorms could potentially occur, a "Widespread (very) severe thunderstorm possible" warning is issued (hatched area on the hazard map).

Thunderstorms can develop very quickly and it is impossible to accurately predict their strength, timing or location several hours in advance. Only when a thunderstorm is present are localised thunderstorm warnings issued at short notice (up to one hour in advance).

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MeteoSwiss is the Federal Office for Meteorology and Climatology. We keep our finger on the pulse of the weather and climate, so it’s all clear as day to you.

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