Nimbus

A cloud is the hardest easy thing to draw. It has no edge to outline and no surface to shade, because it is not an object at all but a region of air where enough water has gathered to get in the way of light. So this one is not drawn; it is integrated. For every point on the screen a ray is sent into a volume of drifting noise, and stepped forward through it, and at each step it loses a little of its light to the density it is passing through, by the same law that dims a torch in fog.

That alone would give you a grey smudge. What makes it read as a cloud is the second march. At each step along the view ray, another short ray is sent toward the sun, to ask how much cloud stands between this spot and the light. Where little does, the spot is bright; where a whole shoulder of cloud is in the way, it falls into shadow. From that one question, asked a few hundred thousand times a frame, the tops come up sunlit and the bellies go dark and a bright seam appears wherever the light is grazing through a thin edge, and the thing stops being smoke and becomes weather.

Because none of this is a fixed shape, you cannot touch it. What you can move is the light. Drag across the cloud and you are steering the sun, and the cloud answers by turning a different face to you, the same body lit from a new side. That is the truest thing about a cloud, and maybe about any medium: it has no features of its own. It is only ever what the light passing through it makes of it. Companion to Reflection #162 and Letter #105 To the One Who Moved the Light.