How Heat, Moisture and Wind Make Clouds and Lightning
Two columns on the physics of the air: rising vapour, thunderbolts, and straight-line wind
Two columns of text set out Leonardo's meteorology: heat lightens moisture so it rises to the cold middle region of the air, where fire trapped in a cloud concentrates, breaks it at its weakest point, and issues as lightning and wind. He argues that wind, like a rocket driven by fire, needs air to resist it, and that it travels in a straight line rather than in a circle as Aristotle held. Closing aphorisms hold that heat disaggregates and dissolves while cold congregates and coagulates. Faint untranscribed sketches appear along the lower edge.
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Fire in a cloud breaks it into lightning and wind
Heat makes moisture lighter than air, so it rises to the cold middle region; there the fire flees to the centre, separates moist from dry vapour, kindles and vaporizes the surrounding moisture, and the growing vaporization constrains the fire until it gains power, breaks the cloud at its weakest part, and opens a door for the motion of the thunderbolt and wind.
Wind moves in a straight line and needs air to resist it
Wind caused within the air can only move if it leans on something that resists it on the side opposite its motion, as with rockets driven by fire against the air that resists and flees more slowly. Leonardo insists the wind moves in a straight line, not a circle as Aristotle would have it, citing the swell of the sea that keeps its straightness even when there is no wind.
Heat disaggregates, cold coagulates
Closing notes state that clouds are composed of distinct globules separated by spaces, and that the motions of the air are born from the resolution and congregation of moisture. The governing aphorism: heat disaggregates and dissolves, while cold congregates and freezes, or rather coagulates.