To take a new approach to the lower thermosphere (80-100km) long-term behavior, the variations of the dates of the equinoctial rearrangements of the zonal wind circulation were considered. We have evaluated the duration of unstable equinoctial rearrangements periods and duration of rather stable summer and winter circulation periods as well. The identical multiyear radio measurements of the horizontal winds in the lower thermosphere/ionosphere at two midlatitude (52¿N) observatories (Irkutsk (Badary Observatory), East Siberia, and Collm, central Europe) during 1981--1996 were used. It is shown that there are significant nonzonality and regional differences in the character of the circulation rearrangements. The duration of the winter circulation in East Siberia is systematically longer than in central Europe. However, the tendency to a decrease of the summer circulation duration for the years under consideration is observed for both regions.