The nature of interactions between surface winds and sand surfaces that can be characterized by aerodynamic roughness length has important implications for aeolian sediment transport. Here we report the wind tunnel results of systematic tests of the aerodynamic roughness lengths of different sized fixed sand beds at different free-stream wind velocities. The results suggest that there are complex variations of aerodynamic roughness length. Wind is an active factor in determining the aerodynamic roughness length of fixed sand beds. The so-called 1/30 law in aeolian research proposed by Bagnold more than a half century ago for sand surfaces is found to suffer some limitations. Some multivariate models relating aerodynamic roughness length and other factors are provided. The differences between the aerodynamic roughness of fixed and mobile beds are discussed, and it is implied that mobile surfaces adapt to the wind by changing roughness. The similarities between aerodynamic roughness and hydraulic roughness are compared. Aerodynamic roughness is divided into aerodynamically smooth, transitional, and completely rough regimes. ¿ 2001 American Geophysical Union