Instrument meteorological conditions
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Instrument meteorological conditions (IMC) is an aviation term that describes weather conditions that normally require pilots to fly primarily by reference to instruments, and therefore under Instrument Flight Rules (IFR), rather than by outside visual references under Visual Flight Rules (VFR). The weather conditions required for flight under VFR are known as Visual Meteorological Conditions (VMC). IMC and VMC are obviously mutually exclusive. The boundary criteria between VMC and IMC are known as the VMC minima.
With good visibility, pilots can determine the attitude of the aircraft by utilising visual cues from outside the aircraft, most significantly the horizon. Without such external visual cues, pilots must use an internal cue of attitude, which is provided by gyroscopically-driven instruments such as the Attitude Indicator (or "Artificial Horizon"). The availability of a good horizon cue is controlled by meteorological visibility, hence minimum visibility limits feature in the VMC minima. Visibility is also important in the avoidance of terrain.
Since the basic traffic avoidance principle of flying under Visual Flight Rules (VFR) is "see and avoid", it also follows that distance from cloud is an important factor in the VMC minima: as aircraft in cloud cannot be seen, a buffer zone from cloud is required.
ICAO recommends the VMC minima internationally; they are defined in national regulations, which rarely significantly vary from ICAO. the main variation is in the units of measurement as different states often use different units of measurement in aviation. The criteria tend to be stricter in controlled airspace, where there is a lot of traffic therefore greater visibility and cloud clearance is desirable. The degree of separation provided by Air Traffic Control is also a factor: for example in Class A and B airspace where all aircraft are provided with standard separation, the VMC minima feature visibility limits only, whereas in classes C-G airspace where some or all aircraft are not separated from each other by Air Traffic Control, the VMC minima also feature distance from cloud minima.
It is important not to confuse IMC with Instrument flight rules (IFR) -- "IMC" describes the actual weather conditions, while "IFR" describes the rules under which the aircraft is flying. Aircraft can (and often do) fly IFR in clear weather, for operational reasons, or when flying in airspace where flight under VFR is not permitted; indeed by far the majority of commercial flights are operated solely under IFR.
