On Clouds: Their Formation, Light, and Redness
Opening of Part Seven of the Treatise on Painting: mists raised by the sun, lit like dense mountains.
This page opens Part Seven of the Treatise on Painting, headed 'On Clouds.' Leonardo explains that clouds are mists drawn up by the heat of the sun and condensed by the cold of the middle region of air, so that their globes take on defined surfaces and shadows like dense mountains lit by solar rays. He then treats why clouds sometimes appear brilliantly illuminated and sometimes wholly dark, and begins his account of the redness they take on when the sun stands at the horizon at evening or morning. The section continues onto the following folios; the page carries text only, with no diagrams.
On this page
How clouds form: mists raised and condensed
Clouds are mists drawn up on high by the heat of the sun, rising until their acquired weight equals the power of their mover. That weight comes of their condensation, which arises from the heat infused in them fleeing the cold middle region of the air. The moisture follows the heat toward the middle of each cloud-globe, which then condenses into defined surfaces like dense mountains.
Clouds lit like mountains, or left wholly dark
Clouds sometimes appear to receive the solar rays and to be illuminated after the manner of dense mountains. At other times the same clouds remain very dark, without the darkness varying in any part. This happens through the shadows cast by other clouds that interpose themselves between the sun and the darkened clouds, taking the solar rays away from them.
The redness of clouds at evening and morning
The redness with which clouds are tinged, greater or lesser, arises when the sun is at the horizons in the evening or the morning. A body with some transparency is somewhat penetrated by the solar rays at those hours, so that the thinner edge-parts of the clouds glow with red. The passage breaks off as it turns to those parts of the clouds nearest their edges.