Visual meteorological conditions
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In aviation, visual meteorological conditions (or VMC) are those in which visual flight rules (VFR) flight is permitted—that is, conditions in which pilots have sufficient visibility to fly the aircraft maintaining visual separation from terrain and other aircraft. They are the opposite of Instrument Meteorological Conditions (IMC). The boundary criteria between IMC and VMC are known as the VMC minima.
Visual meteorological conditions are usually defined by certain visibility minimums, cloud ceilings (for takeoffs and landings), and cloud clearances.
The exact requirements vary by type of airspace, whether it is day or night (for countries that permit night VFR), and from country to country. Typical visibility requirements vary from one statute mile to five statute miles (many countries define these in metric units as 1,500m to 8km). Typical cloud clearance requirements vary from merely remaining clear of clouds to remaining at least one mile away (1,500m in some countries) from clouds horizontally and one thousand feet away from clouds vertically.
Generally, VMC requires greater visibility and cloud clearance in controlled airspace than in uncontrolled airspace. In uncontrolled airspace there is less risk of a VFR aircraft colliding with an IFR aircraft emerging from a cloud, so aircraft are permitted to fly closer to clouds. An exception to this rule is class B airspace class, in which ATC separates VFR traffic from other VFR traffic and from IFR traffic, which is why in Class B Airspace lower cloud clearance is required.
