With Prandtl's theory of the mixing length as the point of beginning, a theory concerning the structure of the atmospheric surface layer is proposed on similar assumptions as were put forward by Lettau. The novel feature in the present treatment lies in the fact that the acceleration due to the frictional part of the turbulence is considered to be dependent on stability, whereas Lettau assumed a constant value for this acceleration. Although this theory is not exact, it may promote a better understanding of atmospheric turbulence. A dimensionless stability number is introduced; it enables one to obtain a simple survey of all states of the atmospheric surface layer. The theory is tested with observations of Rider (1954). The requirements for a further experimental program are established.