Summary - The phenomenon of the green flash at sunset (or sunrise) is renewed.Several possible mechanisms that have been put forth to explain the green flash are discussed. A quantitative model for the phenomenon is then described, which includes parameters that are representative for a polar atmosphere (low humidity ant small aerosol optical depth). lt is supposed that the primary mechanism responsible for causing the green flash are natural molecular dispersion and the filtering action imposed by the atmosphere for low elevation angles. Results from the model indicate that a green rim of verical extent ~ 0.15 milliradians would appear at the upper limb of the sun during sunset or sunrise. The theoretical results are compared with observations of a green flash made at wintertime in interior Alaska.