Periodic variations of scattered twilight according to observations at Abastumani.

A harmonic analysis is applied to monthly mean spectral intensities of scattered twilight observed at solar zenith distances of 98 to 99 deg for an earth-shadow height of 50 km and at solar zenith distances of 100 to 102 deg for an earth-shadow height of 80 km. Annual, semiannual, and four-month periodic variations in the spectral regions centered at 590, 630, and 660 nm are established for both morning and evening twilight. It is shown that the seasonal variation in scattered twilight is well described by the first three components of a Fourier series, that the amplitudes of the annual and semiannual components vary significantly as a function of wavelength, and that all three components of the periodic variation are somehow related to temporal and spatial variations in atmospheric scattering power.