'Yellow Clouds' of Mars
Clouds on Mars are of two main types. The high altitude clouds, described previously, are scattered and of small extent. The so-called 'yellow clouds' are entirely different and may at times cover vast areas of the planet. There were major obscurations in 1971, 1956, and 1924.
There is no suggestion that the obscurations are due to clouds of a terrestrial type, and they are usually attributed to dust or sand storms. They may develop quite quickly, and persist for several weeks or even months, so that the disk of the planet appears more or less featureless. This was the case in late 1971, from early October through to the end of the year, so that when Mariner 9 first went into orbit around Mars the program of photographing the surface had to be postponed.
From Earth it is difficult to learn much about the height of the dust layer, but reliable information has come from Mariner 9. It seems that in 1971 the dustextended almost to the top of Nix Olympica and Nodus Gordii, the highest known points on Mars, which attain an altitude of over 20km. The obscuring effect is virtually total, and as the affected areas are extensive there must be a considerable amount of dust or sand in the atmosphere during a major storm. The problem is: how can it get there?