AbstractTurbulence data collected in an area of complex terrain using a number of instrument packages attached to the tether cable of a captive balloon are presented. Eddy correlation measurements of the turbulent momentum flux are used to determine the magnitude of the roughness length from the drag coefficient and also to show that the roughness length is a useful concept in parametrizing the surface stress in such areas. the relationship between the terrain characteristics and the roughness length is examined using a simple formula, results from a numerical model and previous experimental estimates of the roughness length for complex terrain. It is found that a formula based on numerical model results for separated flows gives reasonable predictions and that the measured roughness length is representative of an area of order 100 km2.Non‐dimensionalized turbulence data are compared with similar data obtained over flat homogeneous surfaces. This comparison suggests that the structure of the turbulent boundary layer over complex terrain is remarkably similar to that over flat homogeneous surfaces. Because of the limitations of fixed point measurements in complex terrain, results from a numerical study of turbulent flow over two‐dimensional sinusoidal orography are shown to indicate the magnitudes of the perturbations introduced into the boundary‐layer flow by large undulations in the underlying surface. the model results demonstrate the validity of the analysis carried out with the