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VFR / VMC visibility requirements in the US. In aviation, visual meteorological conditions (VMC) is an aviation flight category 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.
The international definition of fog is a visibility of less than 1 km (3,300 ft); mist is a visibility of between 1 km (0.62 mi) and 2 km (1.2 mi) and haze from 2 km (1.2 mi) to 5 km (3.1 mi). Fog and mist are generally assumed to be composed principally of water droplets, haze and smoke can be of smaller particle size.
Spatial disorientation is the inability to determine position or relative motion, commonly occurring during periods of challenging visibility, since vision is the dominant sense for orientation.
The opposite property of translucency is opacity. Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of cesia in an order system with three variables, including transparency, translucency and opacity among the involved aspects.
Visibility conditions under an overcast sky between an observer and distant feature are the best because of the lowest level of light scattering by aerosols and air molecules. Quite the opposite situation occurs at twilight when twilight wedge becomes visible.
Circumpolar star trails in a long-exposure photo of several hours. The stars near the celestial pole leave shorter trails with the long exposure. A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets below the horizon due to its apparent proximity to one of the celestial poles.
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Northern Hemisphere circumpolar stars around Polaris, with a long-exposure producing a star trail photo. A pole star is a visible star that is approximately aligned with the axis of rotation of an astronomical body; that is, a star whose apparent position is close to one of the celestial poles.