The Da Vinci Notebooks at sacred-texts.com
If the sun is in the East the smoke of cities will not be visible in the West, because on that side it is not seen penetrated by the solar rays, nor on a dark background; since the roofs of the houses turn the same side to the eye as they turn towards the sun, and on this light background the smoke is not very visible.
But dust, under the same aspect, will look darker than smoke being of denser material than smoke which is moist.