The role of stability and roughness changes in producing diurnal cycles of wind speed offshore is examined using experimental data and numerical simulations. Overnight, the transition from stable conditions over land to less stable conditions over sea combined with the lower roughness of the sea surface produces a marked increase in wind speeds offshore. However, during the day the transition from unstable conditions over land to stable conditions offshore means that the surface layer becomes decoupled from higher wind speeds aloft and the wind speed increase in the surface layer is smaller. Thus, in some cases the diurnal cycle of wind speeds offshore is inverted with respect to that at a typical land site; that is, highest wind speeds occur at night. ¿ American Geophysical Union 1996