Monochromatic extinction coefficients at four wavelengths have been obtained over a period of more than two years at the Observatorio del Teide (Izaña Tenerife) using a full disc, direct sunlight, quadruple photometer devoted to the detection of integral luminosity oscillations of the Sun. The mean extinction coefficients (0.13 at 500 nm) show a seasonal variation of about 15%, the best atmospheric conditions being in winter and autumn. Moreover, in anyone day the extinction coefficient in the afternoon is always lower than the one in the morning by ≡7%. A one-year period fluctuation, with an amplitude of ≡0.035 mag, has been identified in the instrumental magnitudes outside the atmosphere, and is interpreted as the variation produced by the different Sun-Earth distance from winter to summer.