Vol. I: VIII. Botany for Painters and Elements of Landscape Painting: 459.
The Da Vinci Notebooks
When the sun is covered by clouds, objects are less conspicuous, because there is little difference between the light and shade of the trees and of the buildings being illuminated by the brightness of the atmosphere which surrounds the objects in such a way that the shadows are few, and these few fade away so that their outline is lost in haze.