The objective of this paper is to present a method that qualifies the degree of visibility of an offshore wind farm from an observer located along the coast. In many cases, the deployment of an offshore wind farm leads to public opposition. This entails the need for the development of appropriate methods that might present in the most intelligible way the impacts of an offshore wind farm. Amongst many factors to take into account, the visual impact of such farms is surely a factor to take into account. We introduce a visual operator that integrates several parameters that mainly depend on the distance of the wind farm to the coast. We apply a measure that evaluates the horizon surface impact modulated by the number of distinguishable turbines and an aesthetic index based on turbine alignments. The whole method is implemented on top of Geographical Information System (GIS) and provides a decision-aid mechanism oriented to decision-makers. The whole approach is experimented in the context of a wind farm in North West France.