This study focuses on the perceptual processes in situations where sight conditions are restricted due to fog during landing approaches. The H-hypothesis explains the functional utility of the horizon to control the glide angle to a runway visually. Under foggy conditions, horizon information is available both from the explicit (visible) and from the implicit horizon (invisible, but perceptually inferable true horizon). Based on an ecological analysis of the optics during a runway approach under foggy conditions, the functional utility of the explicit and implicit H-angle was tested as a Gibsonian invariant controlling the visual glide slope. N=40 participants viewed simulated landing approaches with varying glide slopes in which sight distances were limited to resemble foggy conditions. Results indicate that subjects rather referred to the explicit than the implicit horizon when estimating the glide angle. Furthermore, there was no connection between participants’ accuracy of the estimated implicit H-angle and the reliability of their glide angle judgements.