The theory of gravity wave-driven fluctuations in the OH nightglow from an extended source region is generalized to account for effects of eddy kinematic viscosity &ngr; and eddy thermal diffusivity &kgr;. In the nondiffusive case, the amplitudes and phases of vertically integrated normalized intensity ⟨ΔI⟩/⟨I¿⟩ and temperature ⟨ΔTI⟩/⟨T¿I⟩ perturbations and vertically integrated Krassovsky's ratio ⟨&eegr;⟩ as functions of period are influenced by the upper limit of vertical integration of the extended source, especially at long periods when vertical wavelengths &lgr;v, are small. The effects, which include oscillations in ⟨ΔT⟩/⟨I¿⟩, ⟨ΔTI⟩/⟨T¿I⟩, and ⟨&eegr;⟩, particularly at long periods, are due to constructive and destructive interference of nightglow signals from vertically separated levels of the OH emitting region that occur when &lgr;v is comparable to or smaller than the thickness of the main emission region. The sensitivity of these ratios to the upper limit of vertical integration occurs because of the relatively small rate of decay of the intensity of OH emission with height above the peak emission level and the exponential growth with altitude of nondissipative gravity waves. Because eddy diffusion increases &lgr;v, especially at long periods, and reduces wave growth with height compared with the case &ngr;=&kgr;=0, inclusion of eddy diffusion removes the sensitivity of ⟨&eegr;⟩ and the other ratios to the maximum height of vertical integration. It is essential to account for both eddy diffusion and emission from the entire vertically extended emission region to correctly predict ⟨&eegr;⟩, ⟨ΔI⟩/⟨I¿⟩, and ⟨ΔTI⟩/⟨T¿I⟩ at long gravity wave periods. Observations of Krassovsky's ratio are consistent with theoretical predictions of ⟨&eegr;⟩ for horizontal wavelengths larger than about 500 km, provided diffusion and emission from an extended region are both taken into account. ¿ American Geophysical Union 1991