Definition
The sound produced by the rapid expansion of air that has been suddenly heated by a lightning discharge. The intense heat of the lightning channel causes the surrounding air to expand explosively, creating a shock wave that is heard as thunder.
Plain English
The loud noise that follows lightning. Lightning heats the air around it so fast that the air bursts outward, and that burst is what you hear.
Context Anchor
Pilots encounter thunder in weather reports, preflight weather briefings, and real-time decisions about avoiding thunderstorms.
Derivation
From the Old English thunor, related to the Norse god Thor. The word has always referred to the sound itself, not the lightning that causes it. Useful to remember: thunder is only the sound — the lightning is the actual hazard.
Why Pilots Care
Thunder indicates active thunderstorms, which bring turbulence, hail, wind shear, and lightning hazards that pilots must avoid.
Grounding Statement
When lightning flashes nearby, the air is heated almost instantly, expands violently, and the resulting sound wave reaches you as thunder.
Intuition Check
Thunder is not the dangerous part by itself. It is a warning sign that lightning and thunderstorm conditions are nearby.
Example Sentence 1
The pilot delayed the preflight when thunder was heard in the distance, indicating a thunderstorm was within striking range of the airport.
Example Sentence 2
Thunder in the distance prompted the crew to review the latest convective SIGMET before departure.