1. A detection method for detecting an interference phenomenon (F) creating atmospheric interference so as to cause a loss of visibility for a user of a motor vehicle (V), comprising the steps of: determining a region of interest (ROI) in an acquired image (I) of an environment (E) of said motor vehicle (V), said region of interest being vertical and occupying parts of said acquired image (I) on each side of a horizon line (H); determining a grey-level curve (CL), taking into account only the median value of the grey levels of the pixels situated in said region of interest (ROI) of said acquired image (I): wherein said detection method also comprises the steps of: calculating at least one tangent (TG) from a point of interest (PT) in said grey-level curve (CL), said point of interest (PT) being situated above said horizon line (H); determining a directing coefficient (CF) from said at least one tangent (TG); deducing the presence of said interference phenomenon (F) from the value of said directing coefficient (CF) determined.