Abstract: Apparatus for estimating the slant visibility in obscured atmospheres such as fog, whereby an indication is derived as to when, during an aircraft landing approach in such conditions, the aircrew are likely to be able to see the runway lights. The apparatus comprises a transmissometer array arranged to take visibility readings over a slanted distance from different heights not far from a runway threshold, and a computer arranged to compare the readings with a plurality of fog profile models, to determine the definitive model and hence to derive and indicate a visual acquisition estimate.