Mountain waves must be parameterized in numerical models of the atmosphere, but long-term observations of their characteristics are sparse. In this study, a large data set from the VHF radar at Aberystwyth (52.42 ¿N, 4.00 ¿W), during the years 1990--1998, is used to investigate the relation between background wind direction, local topography, and the mountain wave alignment. The horizontal wave vector of the mountain waves is, on average, found to be biased 10¿ anticlockwise from the wind direction in the lower troposphere near 2 km altitude, and biased 20¿ clockwise from the wind direction near the ground. This can be explained if the wave-launching height (where the background wind speed and direction values can be assumed responsible for generating the waves), lies within the boundary layer, where the wind has only partly rotated between its direction near ground level and its direction in the free troposphere above the boundary layer. On average, and for most background wind directions at Aberystwyth, the wind direction within the boundary layer is a more important influence on wave alignment than the orientation of upstream mountain ridges. The relative height of the mountain peaks and the boundary layer may affect the average alignment of mountain waves and therefore the wave stress by as much as 30¿. ¿ 1999 American Geophysical Union