More than 2000 mean night temperatures (1987--1995) were derived from OH* near-infrared emissions in the upper mesosphere (around 87 km) above Wuppertal (51 ¿N, 7 ¿E) from ground-based measurements. Variations of 4--50 days' period were analyzed using maximum entropy and wavelet methods. A climatology showing the seasonal dependence of occurrence and long-term evolution of the strength of these variations is presented. While longer period oscillations (greater than ~10 days' period) are observed to be more frequent during local winter and almost absent during local summer. There is an opposite behavior for the shorter period oscillations. Temperature oscillation amplitudes are found to be modulated with a quasi 2 year oscillation. It is believed that the quasi-biennal oscillation is responsible for these modulations. A long-term trend showing an increase of about 16% per decade is found in the temperature variability. Furthermore, positive and negative long-term trends are found in the temporal evolution of temperature oscillation amplitudes. A possible relationship to solar activity is investigated. Evidence is found that shorter period oscillations (smaller than ~20--25 days) are anticorrelated while longer period oscillations are correlated with solar activity. ¿ 2000 American Geophysical Union