Despite the impact it has on human activity, particularly transport, accurate forecasting of fog remains a major challenge for numerical weather prediction models. The complex interaction between various physical processes, many of which are parametrised and highly sensitive to small changes, is one of the key reasons for poor fog forecasts. One challenge for numerical models is predicting the structure of the boundary layer, which often undergoes a transition from statically stable to weakly unstable during the life cycle of a fog event. The recent local and non‐local fog experiment (LANFEX) has provided a new comprehensive and detailed observational dataset of fog events. Here, a case study has been used as the basis for an investigation of the effect of the humidity of the residual layer and wind speed on the stability of the boundary layer during a fog event. We find a very high sensitivity in the timing of the stability transition during the fog event; for example, a +3% relative humidity perturbation results in a delay of almost 3h, while a 0.45ms−1 10m wind speed perturbation results in a delay of more than 8h.