The degree of the blue clearing at Syrtis Major in the 1982 apparition shows dual components of variations. The phase-dependent part increases as Mars approaches the opposition, and the diurnal component reaches a maximum near local noon at Syrtis Major. The phase-dependent part is attributed to the opposition effect of the surrounding bright areas, and the diurnal variation is due to morning and evening clouds in the tropics. The opacity of the morning cloud is approximately unity in the early morning and decreases to null at about local noon. ¿American Geophysical Union 1990