A thunderstorm is termed severe if one or more features are present. A severe thunderstorm is potentially capable of causing significant damage.
A thunderstorm that has one or more of the followng is termed "severe":
(1) wind speed > 58 miles per hour (> 93 km per hour)
(2) hail is formed AND is > 1 inch (2.54 cm) in diameter
(3) funnel cloud and/or tornado formed
In Canada a fourth criteria is used:
(4) rainfall > 2 inches (> 5.08 cm) in 1 hour OR > 3 inches (> 7.6 cm) in 3 hours
Although lightning is a feature of all thunderstorms and can be deadly it is not one of the criteria for a severe thunderstorm.
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Specialty: Emergency Medicine, Critical Care
