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-22- I I I I I I I I I I I I I I I I I I II 8PECTEAL CHARACTERISTICS OF RELEASE SITE WINDS The release-site winds during diffusion test periods were read every five seconds and analyzed by the power spectrum method. For each te st, the mean wind direction and speed, the total spectral energy of. the wind direction and of the wind speed, the normalized spectral density at selected frequencies (to depict the shape of the curves), and the maximum coherence between speed and direction and the frequency at which this occurs were computed. The total spectral energy and the spectral densitie s at various frequencie s were then examined for re- lationships to wind speed and wind direction. If mechanical turbulence is a major contributor to the observed spectral energy, it should be related to wind speed and possibly wind direction, because surface roughness and topography vary with direction. The total spectral energy of both wind direction and wind speed were not significantly related to mean wind speed or mean wind direc- tion, The spectral density at a high and a low frequency (0,05 and. 01 cps) were then examined for a relationship to wind speed. The most significant relationship found was between the wind direction and wind speed spectral content at .05 cycles per second, and this is shown in Figure 8. The result agrees with the findings of others, that the high frequency end of the spectra for lateral and longitudinal velocity components is controlled by wind speed (Lumley and Panofsky, 1964). Mechanical turbulenc e should have an effect on the low frequency part of the longitudinal velocity spectrum, but not on the lateral velocity spectrum. The major effect on the lateral velocity spectrum should be produced by stability. The computed spectra were ranked by stability and examined for differences in shape. In general, the unstable cases had proportionally more energy in the lower frequencies, and the coher- ence between the wind direction and the wind speed was higher at all frequencie s for the unstable case s.