The incoherent scatter radar at Arecibo has been used to measure the 80- to 100-km wind system on two November and two April days. The wind system is shown to correspond most closely to the S1,1 mode diurnal tide. Dissipation effects were always important, and in the April data the tide was seen to 'break' at an altitude of 92 km and an amplitude of 60 m/s. The observed vertical wavelength was &lgr;z=19.5 km; a Richardson number of 0.2, a 40¿K temperature oscillation, and upward energy flux of 4.2¿10-4 W/m2 inferred.