We present vertical wave number spectra of wind fluctuations using data with a height resolution of 25 m observed near the mesopause by 64 chaff rockets. Direct measurement of Brunt-V¿is¿l¿ frequency allows accurate calculations of spectral amplitude and local Richardson number. Individual vertical wave number spectra reveal considerable variability in both slope and amplitude, which is not consistent with the predictions of various saturation models. Mean vertical wave number spectra observed at different locations and seasons show great similarities. While the spectral amplitude predicted by the linear saturation model or saturated-cascade model is comparable to the spectral amplitude of the observed mean spectrum, those spectral amplitudes predicted by other saturation models are larger than the observed mean amplitude within a factor of 3. However, mean vertical wave number spectra observed at 69¿N exhibit significant seasonal variation with a shallower slope of -2.40 and a larger spectral amplitude of 2.6 ¿ 105 (m2/s2)/(cyc/m) at m = 1/(4 km) in winter compared to that spectrum observed in summer. Very large mean wind shears, as high as 42--77 m/s/km, are observed to be present at several heights near the mesopause. Such strong mean shear profiles, together with the Brunt-V¿is¿l¿ frequency squared N2 profiles, act collectively to produce dynamical instability regions of local Richardson number Ri smaller than 0.25. These regions are found to correlate well with the saturated gravity wave spectra. In contrast, the stable region of local Richardson number Ri larger than 0.4 observed at high latitude in winter is found to associate with an unsaturated gravity wave spectrum.