Determining Visibility

Visibility is of great importance to pilots who wish to determine if VFR flight is possible or not, whether it is for take-off, landing or route flight. Visibility is either assessed, for example by control tower personnel who know the distance from their observation point to a landmark, or measured by a device called a transmissometer located on a runway airport which is capable of measuring the visibility along the runway.

One of the problems encountered by pilots is the visibility looking forward and down from an aircraft in flight. This is termed oblique and is measured from a point on the ground immediately below the aircraft along the surface to the furthest point that the pilot can see. Oblique visibility varies depending on the height and location to the aircraft.

Inflight visibility changes as the angle of view vary. For example, when overflying an aerodrome, the runway complex may be clearly visible through fog or mist but as slant angle decreases the runway may become obscured. The figure below depicts how high slant angles determine whether a pilot sees through less fog or mist than would be the case with low slant angles.

On other occasions, a layer of the cloud may reduce air to ground visibility while the horizontal visibility on the airfield is good.

Air to air visibility may be reduced by cloud, precipitation, dust, smoke, and haze. For instance, pollutants may be trapped under inversion and reduce visibility. Flying above the inversion, at A in the Figure below, will ensure good forward visibility while flying within the inversion at B, will drastically reduce forward vision. A pilot at A may not be able to see the ground.

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