Definition
A thunderstorm classified as severe when it produces one or more of the following: surface wind gusts of 50 knots (58 mph) or greater, hail at the surface three-quarters of an inch in diameter or larger, or tornadoes.
Plain English
A thunderstorm strong enough to be officially called severe because it has very strong wind gusts at the ground, large hail, or tornadoes. Ordinary thunderstorms are dangerous to aircraft, but severe thunderstorms meet a specific threshold that makes them especially destructive.
Context Anchor
Pilots encounter this term in aviation weather briefings, weather radar discussions, convective weather products, and go/no-go decisions before flight.
Derivation
Thunderstorm combines “thunder” and “storm,” pointing to a storm with lightning and thunder. Severe comes from Latin meaning serious or strict; in aviation weather, it points to a storm meeting specific serious hazard limits, not just a storm that seems intense.
Why Pilots Care
Severe thunderstorms can produce microbursts, severe turbulence, and structural damage that quickly exceed aircraft and pilot capabilities.
Grounding Statement
Picture a thunderstorm strong enough to throw damaging wind across the ground, drop large hail, or produce a tornado; that is the level of hazard meant here.
Intuition Check
Do not read “severe” as a personal impression, like “that storm looks bad.” In this term, “severe” means the storm meets defined wind, hail, or tornado criteria.
Example Sentence 1
The convective SIGMET warned of a line of severe thunderstorms with hail and 60-knot wind gusts moving across the route, so the pilot diverted well around them.
Example Sentence 2
Controllers rerouted traffic to avoid the severe thunderstorm that was producing 60-knot winds near the airport.