The vertical motion of ionospheric sporadic layers in the 80- to 140-km altitude region is examined by using a numerical solution for ion trajectories in a wind system composed of diurnal and semidiurnal tides similar to those observed at Arecibo. The confluence of the two tides in the 100- to 110-km altitude region is shown to produce a variety of layer motion possibilities including 'quasi-stagnation' of the layer as well as rapid upward or downward motions. Experimental layer trajectories are presented which confirm the basic theoretical results and which indicate the continuous opportunity for layers to form in the tidal wind system. The availability of metallic ions for layer formation is discussed as is the observed presence of layers below 90-km altitude.