The 4-day wave is a sinusoidal feature in the temperature field of the polar winter stratosphere. Preliminary observations by Venne and Stanford (1979) indicate that it has a zonal wavenumber 1 structure and is eastward propagating with (TURN)4 days period. In this work, additional observational evidence is presented which suggests that the wave represents a normal mode oscillation of the atmosphere. A polar-plane approximation to the earth's sphericity at high latitudes is described and applied to the equations of air parcel motion, yielding solutions that resemble the 4-day wave. In order to obtain these solutions the zonal mean wind is taken to be in solid rotation with no vertical shear, conditions which are typical in the region of largest 4-day wave amplitudes. The substantial changes in the mean zonal wind caused by stratospheric sudden warmings are shown to modify the 4-day wave.; The results of spectral analyses performed on infrared radiance data from the Nimbus satellites are also presented. These spectra, for wavenumbers up to 4, reveal a new wave mode with wavenumber 2 structure and westward motion of period (TURN)2.3 days. This wave, found in the Southern Hemisphere winter stratosphere, is tentatively identified as a normal mode wave.; The distortions present in satellite data of the Nimbus type, caused by the Doppler shifting of wave phase speed by the satellite motion, are discussed. Simple corrections which provide better estimates of wave amplitude are derived. These corrections are applicable when leakage of power from other wavenumbers is not appreciable.
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