In aviation, visual flight rules (VFR) are a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minima, i.e., in visual meteorological conditions (VMC), as specified in the rules of the relevant aviation authority. The pilot must be able to operate the aircraft with visual reference to the ground, and by visually avoiding obstructions and other aircraft. If the weather is less than VMC, pilots are required to use instrument flight rules, and operation of the aircraft will be primarily through referencing the instruments rather than visual reference. In a control zone, a VFR flight may obtain a clearance from air traffic control to operate as Special VFR. Requirements VFR require a pilot to be able to see outside the cockpit, to control the aircraft's altitude, navigate, and avoid obstacles and other aircraft. Governing agencies establish specific requirements for VFR flight, including minimum visibility, and distance from clouds, to ensure that aircraft operating under VFR are visible from enough distance to ensure safety. Under visual meteorological conditions, the minimum visual range, distance from clouds, or cloud clearance requirements to be maintained above ground vary by jurisdiction, and may also vary according to the airspace in which the aircraft is operating. In some countries, VFR flight is permitted at night, and is known as night VFR. This is generally permitted only under more restrictive conditions, such as maintaining minimum safe altitudes, and may require additional training as a pilot at night may not be able to see and avoid obstacles. The VFR pilot is required to "see and avoid" obstacles and other aircraft. Pilots flying under VFR assume responsibility for their separation from all other aircraft and are generally not assigned routes or altitudes by air traffic control (ATC). Depending on the category of airspace in which the flight is being conducted, VFR aircraft may be required to have a transponder to help Air Traffic Control identify the aircraft on radar in order that ATC can provide separation to IFR aircraft. Meteorological conditions that meet the minimum requirements for VFR flight are termed visual meteorological conditions (VMC). If they are not met, the conditions are considered instrument meteorological conditions (IMC), and a flight may only operate under IFR. IFR operations have specific training