1. An apparatus for estimating slant visual range in fog comprising: a transmissometer array, having at least three transmissometer elements at respectively greater differing heights h.sub.1, h.sub.2 and h.sub.3 above the ground and disposed for operation between the differing heights and ground level, for generating transmissometer signals indicative of a fog strength; and a slant visual range (SVR) computer coupled to said transmissometer array so as to receive the transmissometer signals therefrom, said SVR computer having: means for converting the transmissometer signals into signals .sigma.h.sub.1, .sigma.h.sub.2, .sigma.h.sub.3 representing the mean integral of extinction coefficient between said respectively greater heights h.sub.1, h.sub.2, h.sub.3, above ground and ground level; means for storing a predetermined set of extinction coefficient .sigma..sub.h function models, each model characterising a different type of fog pattern, each model having predetermined criteria for its selection and use, and containing at least one empirical constant; means for selecting one of the models in accordance with values of .sigma.h.sub.1, .sigma.h.sub.2, .sigma.h.sub.3 determined by said converting means; means for determining the values of the empirical constant in the selected model thus obtaining a definitive extinction coefficient profile, means for storing values representing airfield lighting and accepting values representing flight path and background brightness, and means for estimating slant visual range from the definitive profile and the airfield lighting, background brightness and flight path values.