50 Years of the Monin-Obukhov Similarity Theory

After Obukhov found in 1946 a universal length scale for exchange processes in the surface layer, a logical consequence was the development of the similarity theory by Monin and Obukhov in 1954. This theory was the starting point for the development of new devices and for several experiments, as well as for the formulation of the presently used universal functions. In the paper, a short historical survey is presented, followed by a description of the present status of the theory, covering topics such as the accuracy of the universal functions, the influence of moisture, and the turbulent Prandtl number. Furthermore, the failure of the theory is discussed, including counter gradients, decoupling, low atmospheric boundary layers, roughness layer, etc. Also presented are the consequences for both experimental approaches and modelling.