Drag this to zero. Single scattering only — every photon allowed exactly one bounce. That's dirty grey smoke, and it's what almost every first cloud renderer produces. Everything white about a cloud is light that bounced more than once.
The height gradient. Low values keep density flat and near the cloud base; high values let it build tall and spread out at the top.
High-frequency noise subtracted from the low-density regions only. Cores stay solid, edges get eaten into wisps.
Pure translation. The field slides past and nothing about it changes.
The boil. Detail noise scrolls upward through the cloud while the base shape walks sideways through the field — so the cloud changes rather than moves. Set wind to zero and this to 2 to see it alone: dead calm air, cloud still churning.
The Henyey–Greenstein phase term. Water droplets throw light strongly forward, which is why clouds glow when you look toward the sun and go flat when it's behind you. This is the silver lining.
Internal render scale. Turn it down if it stutters — volumetrics are expensive and this is running on your phone.
What's running: a ray per pixel marched through a slab of noise. At every step it samples density, then fires a second short ray at the sun to measure how much cloud is in the way. That inner loop is why this costs a hundred times what the water did.
The sun ray gives an optical depth τ. Single scattering turns that into brightness with one exponential, exp(−τσ), and gets smoke. The multiple-scattering slider adds two more exponentials with lower extinction and lower weight — three terms standing in for hundreds of real bounces. It's a cheat with a paper behind it, and it's the entire difference between grey and white.