Wind-profiling Doppler radars measure vertical motions along a vertically-directed beam. By averaging over long periods it is possible to measure very small mean vertical motions. Long-term measurements at Christmas Island have been analyzed to determine the diurnal variation in the vertical motion field. The outstanding feature that emerges from the analysis is a pronounced diurnal cycle in the subsiding motion of the main troposphere (4--10 km) and an equally pronounced diurnal cycle in upward motion centered around the altitude of the tropopause. The possibility of radiative forcing of the observed diurnal cycle is considered in the context of the diurnal variation of atmospheric diabatic heating and cooling rates. ¿ American Geophysical Union 1992