Abstract Evaporation ducts are ubiquitous phenomena over the oceans, and they are responsible for much of the over-the-horizon propagation occurring with millimeter and microwave radars. The height of the evaporation duct depends on meteorological parameters, most notably air and water temperature and relative humidity. In most cases, models that predict the height of the evaporation duct are based on Monin–Obukhov similarity (MOS) theory, and uncertainties in model predictions are often attributed to the simplifying assumptions behind MOS theory. In this paper, an extension to MOS theory is introduced that accommodates spatial variability of meteorological parameters in coastal regions. It is found that for even weakly varying meteorological conditions there are substantial differences in duct heights predicted when spatially varying conditions are invoked versus spatially homogeneous conditions.