Radars have been used successfully for many years to measure atmospheric diurnal and semidiurnal tides, but studies of the atmospheric terdiurnal tide have only rarely been reported to date in the literature. In this paper we investigate the amplitude, phase, and vertical wavelength for the terdiurnal tide using the University of Western Ontario (UWO) 2 MHz radar near London, Canada. Emphasis is placed on short-term, monthly, seasonal, and annual variations of the terdiurnal tide. The results of the present study bring out certain new aspects of the terdiurnal tide at midlatitudes in the northern hemisphere. We show that terdiurnal tides have significant amplitudes in the mesosphere and lower thermosphere; indeed, the terdiurnal amplitudes are often comparable to the diurnal ones. We also draw attention to the annual variability and especially highlight the occurrence of the terdiurnal tide in nonwinter months. For example, the occurrence of the terdiurnal tide in spring months is not uncommon. Thus it seems that the 8-hour component observed over London, Canada, cannot be explained by the terdiurnal tide being due to solar heating alone. Particular cases of the terdiurnal tide are examined, and the observed amplitude and phase profiles of the terdiurnal tide at certain times of the year are discussed in terms of nonlinear interaction between the diurnal and semidiurnal tides. We suspect that this effect may also explain the fact that diurnal and semidiurnal tides at our station often seem suppressed relative to other sites <Thayaparan et al., 1995a>. On the basis of our previous and present observations, it is also possible that large terdiurnal amplitudes observed at certain times of the year may be partly explained in terms of tidal/gravity wave interactions. However, this possibility needs to be further investigated in the future.¿ 1997 American Geophysical Union