Editing Instrument meteorological conditions (section)

==Distinction from Visual Meteorological Conditions==
The weather conditions required for flight under VFR are known as [[visual meteorological conditions]] (VMC). The boundary criteria between VMC and IMC are known as ''VMC minima''. IMC and VMC are mutually exclusive. In fact, instrument meteorological conditions are defined as less than the minima specified for visual meteorological conditions.<ref>{{cite web|url=http://www.faa.gov/air_traffic/publications/atpubs/PCG/index.htm|title=Pilot/Controller Glossary|access-date=2009-09-21|url-status=dead|archive-url=https://web.archive.org/web/20090927115927/http://www.faa.gov/air_traffic/publications/atpubs/PCG/INDEX.HTM|archive-date=2009-09-27}}</ref> Conditions that are above VMC minima but relatively close to one or more of them are sometimes referred to as '''marginal VMC''', and flight in such conditions is referred to as ''marginal VFR''.<ref name="AOPA-1">{{cite web |title=Spatial Disorientation Accidents: VFR in VMC |url=https://www.aopa.org/training-and-safety/online-learning/safety-spotlights/spatial-disorientation/spatial-disorientation-accidents-vfr-in-vmc |website=[[AOPA]] |date=30 July 2020 |access-date=15 July 2023}}</ref>

[[File:US airspace visibility 2016.jpg|thumb|right|upright=1.5|Summary of United States VMC minima ({{CodeFedReg|14|91|155}}), providing requirements for minimum visibility and separation from clouds for different [[airspace class]]es. In this example, for airspace classes C/D/E up to an altitude of {{convert|10000|ft|m|abbr=on}} [[above mean sea level|AMSL]], whether day or night, VMC minima require {{convert|3|mi|km|0}} of visibility (denoted "3&nbsp;SM") and cloud clearance of {{convert|1000|ft|m|abbr=on}} above, {{convert|500|ft|m|abbr=on}} below, and {{convert|2000|ft|m|abbr=on}} horizontally ("152").]]
[[ICAO]] recommends the VMC minima internationally; they are defined and enforced by national regulations, which rarely significantly vary from ICAO. The typical variation is in the units of measurement as different regulatory authorities use different units of measurement in aviation.

The VMC minima tend to be stricter in controlled airspace, where there is increased air 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 strictly-controlled [[airspace class|class A and B airspace]], where all aircraft are provided with positive 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 cloud separation criteria.

===Visibility and separation from clouds===
With good visibility, pilots can determine the [[aircraft attitude]] by utilising visual cues from outside the aircraft, most significantly the [[horizon]]. Without such external visual cues, pilots may be subject to [[Sensory illusions in aviation|sensory illusions]] and must use an alternative reference for the attitude, which is usually provided by gyroscopically-driven instruments such as the [[attitude indicator]] ("artificial horizon"). The availability of a good horizon cue is controlled by meteorological visibility, hence minimum visibility limits feature in the VMC minima. 

Because the basic traffic avoidance principle of flying under [[visual flight rules]] (VFR) is to "see and avoid", it follows that distance from clouds is an important factor in the VMC minima: as aircraft flying in clouds cannot be seen, a buffer zone from clouds established by the minimum separation requirements provides for time to react to an unseen/unknown aircraft exiting the clouds, especially when air traffic control may not be enforcing aircraft separation (as in airspace classes C-G).

===Use of flight instruments under VMC===
IMC should not be confused with IFR ([[instrument flight rules]]) – 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; for example, in the United States, flight under VFR in class A airspaces is prohibited except in emergencies. Indeed by far the majority of commercial flights are operated solely under IFR.

It is possible to be flying under VFR in conditions that are legally considered VMC, but still be forced to rely on flight instruments for attitude control because there is no distinct external horizon; for example, at night over water, which may create a so-called black hole effect if the sky and ground are equally dark, or when lights on the water cannot be distinguished from stars in the sky.