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.
Danger levels thunderstorms
Danger level 1 (minimal or no danger)
A thunderstorm event that is usual for the time of year (for which, therefore, no special warnings are issued).
Danger level 2 (moderate danger)
A thunderstorm event that is usual for the time of year (for which, therefore, no special warnings are issued).
Danger level 3 (significant danger)
Impact
- Flash flooding of streams.
- Toppling of trees.
- Possibility of localised landslides on steep slopes.
- Damage from hail and lightning strikes.
- Failure of drainage and sewer systems.
- Flooding of underpasses, underground garages and cellars.
- Disruption to road, rail and air traffic.
- Danger to vessels on lakes from very strong gusts of wind arising rapidly without warning.
Recommended action
Information on recommended actions can be found here: What to do during a thunderstorm
Description of measurement and prognostic values
Severe thunderstorm (flash thunderstorm; localised, warning time 0 to 2 hours): 90-120 km/h, 2-4 cm (hailstones), 30-50 mm/h
Danger level 4 (severe danger)
Impact
- Flash flooding of streams.
- Toppling of trees.
- Possibility of localised landslides on steep slopes.
- Damage from hail and lightning strikes.
- Failure of drainage and sewer systems.
- Flooding of underpasses, underground garages and cellars.
- Disruption to road, rail and air traffic.
- Danger to vessels on lakes from very strong gusts of wind arising rapidly without warning.
Recommended action
Information on recommended actions can be found here: What to do during a thunderstorm
Description of measurement and prognostic values
Very severe thunderstorm (flash storm, local; warning time 0 to 2 hours): >120 km/h, >4 cm hailstones, >50 mm/h
Danger level 5 (very severe danger)
Only thunderstorm warnings of level 3 and 4 are issued. Despite the most modern tools and methods, it is not possible at this time to predict and warn of violent thunderstorms precisely in terms of their strength, time and place over several hours in advance. 3 Distinctive levels are therefore of little use.
Notes:
A distinction is made between «severe thunderstorms (widespread)» and «flash thunderstorms»:
- «Severe thunderstorms (widespread)» is the term used for a more extensive region, with a forecast of up to 12 hours, when the potential exists for widespread severe thunderstorms.
- «Flash thunderstorm» is the term used to describe short-term warnings (0 to 2 hours) within a more localised region. This warning is only issued once the intensity of a thunderstorm cell has reached the corresponding category. At least one of the criteria of hail, wind or rain (see table) must be met in order for a particular thunderstorm warning to be issued.