Definition
Weather conditions in which visibility, distance from clouds, or cloud ceiling are below the minimums required for flight by visual reference outside the aircraft. In IMC, the pilot must control and navigate the aircraft using the flight instruments rather than the natural horizon.
Plain English
Weather that is too poor to fly by looking outside. The pilot can no longer see well enough to use the horizon, ground, or clouds for reference, so the aircraft must be flown by reading the cockpit instruments.
Context Anchor
Seen in training and decision-making discussions, especially when a pilot must decide whether to continue, turn back, delay, or fly under instrument flight rules because the weather no longer supports visual flying.
Derivation
Instrument refers to the cockpit gauges that show attitude, heading, altitude, and speed. Meteorological comes from the Greek meteoron, meaning something in the sky, and is the root of meteorology — the study of weather. So IMC literally describes weather conditions that force the pilot to rely on instruments.
Why Pilots Care
Determines whether a pilot must operate under instrument flight rules, file an IFR flight plan, and maintain instrument currency for legal and safe flight.
Grounding Statement
Picture flying into a cloud or thick haze where the horizon disappears; that is the kind of situation IMC is meant to describe.
Intuition Check
IMC does not simply mean “bad weather.” It specifically means weather below the minimums for normal visual flight, where the aircraft must be flown by reference to instruments.
Example Sentence 1
The forecast showed deteriorating ceilings, so the pilot filed an IFR flight plan in case the route turned to IMC.
Example Sentence 2
Once in IMC the crew transitioned to instrument references and maintained assigned altitudes and headings.