Definition
Weather conditions in which visibility, distance from clouds, or cloud ceiling are below the minimums required for flight under Visual Flight Rules (VFR), requiring the pilot to operate the aircraft by reference to instruments rather than by outside visual cues. Flight in IMC requires an instrument rating and an active IFR flight plan in controlled airspace.
Plain English
Weather that's too poor to fly by looking outside. The pilot has to fly using cockpit instruments because clouds, fog, or low visibility make it impossible to see the ground or horizon clearly.
Context Anchor
Seen in instrument flying, approach procedures, radar assistance, and weather-related ATC communications.
Derivation
From the regulatory term 'Instrument Meteorological Conditions.' 'Meteorological' comes from Greek 'meteoron' meaning 'thing high up' — the same root as 'meteorology,' the study of the atmosphere. The label simply means 'weather bad enough that you must fly by instruments.'
Why Pilots Care
Determines whether a flight operates under visual or instrument rules, directly affecting routing, separation, and the requirement for an instrument rating.
Grounding Statement
If the clouds or visibility prevent the pilot from clearly seeing enough outside to fly visually, the flight is in or near IMC.
Intuition Check
IMC does not mean “bad weather” in a general everyday sense. It specifically means weather below visual-flight minimums, where outside visual references are not enough and instrument flying is required.
Example Sentence 1
The ceiling dropped to 400 feet overcast with two miles visibility, so the field was solidly in IMC weather conditions.
Example Sentence 2
ATC provided altitude assignments to keep the aircraft clear of terrain while flying in IMC weather conditions.