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I must have missed the original post but the reply by David Basey is spot on. The optical depth is important, given the rather thin atmosphere of Mars. Some types of cloud on Mars, like the north polar spiral clouds, are best seen at the morning terminator, whereas the orographic or mountain clouds are best seen in late afternoon and evening. Just occasionally, when the band of equatorial cloud is at its most dense it can be seen in white light or in RGB composite images. A good example is the recent image of Phil Miles from a few days ago posted in the current observations part of the Mars Section website. In blue light the cloud blots out most of the Syrtis Major at the central meridian but only a trace can be seen in the white light composite. Past observers from 100 years ago rarely used colour filters but by looking for comments upon the faintness or obscuration of the Syrtis on the CM we can infer that the ECB formed every Martian year: for example, Denning in 1903 was surprised to see a belt of white cloud cutting across the south part of the Syrtis. Thus the present informs the past, as it always does… or is it vice versa?