We conducted imaging observations of Mars in March 1997 and April 1999; these apparitions were around its northern summer oppositions. Mars showed highly developed clouds that formed the aphelion cloud belt. Comparing the diurnal variations of the apparent contrasts of Syrtis Major to the adjacent bright areas with those of the optical depths of the aphelion clouds over those regions, we conclude that the time when those regions are subject to the morning clouds is around 0900 Martian Local Time (MLT) and that the diurnal variation of the degree of blue clearing strongly depends on the difference between the optical depth of H2O ice clouds over Syrtis Major and that over the adjacent bright area. Moreover, our simple linear regression analyses of the degree of blue clearing to the optical depths of H2O ice clouds suggest that the key region in which the H2O ice cloud optical depth determines the degree of blue clearing is Syrtis Major. ¿ 2001 American Geophysical Union