Oblique visual range
From Glossary of Meteorology
oblique visual range
(Also called oblique visibility, slant visibility.) The greatest distance at which a specified target can be perceived when viewed along a line of sight inclined to the horizontal.
One must distinguish upward from downward oblique visual range because of the quite different background luminance prevailing in the two cases. Furthermore, a range can only be considered with respect to some given type of target, as is also true of the ordinary (horizontal) visual range. In view of the great importance of the downward oblique visual range in air to ground visual contact and the upward oblique visual range in visual detection of aircraft, it is unfortunate that no satisfactory theory for this has yet been developed. The principal obstacle in treating this problem lies in the typically nonuniform height variation of the extinction coefficient.