The behavior of the Priestley-Taylor parameter α is investigated using a simplified atmospheric boundary-layer model coupled to the Penman-Monteith equation. It is found that under conditions typical for a sunny summer day in the mid-latitudes, α is primarily determined by the surface resistance r3, (α ≈ 1.3 when r3 = 0, α ≈ 1 when r3 ≈ 60 s m−1, and α ≈ 0.6 when r3 ≈ 250 s m−1). This is in good agreement with, experimental values reported in literature. The model is an extension of the models by McNaughton (1976) and Perrier (1980).