Three.js Sky, Clouds and Fog

Skybox / environment map: a cubemap or equirectangular HDRI texture set as scene.background (and often scene.environment for lighting too). Fastest to set up, looks as good as the source photo/render, and doubles as realistic image-based lighting if it's an HDRI. The tradeoff: it's static, so there's no time-of-day change without swapping textures or blending between two.

Procedural sky (Sky addon): Three.js ships a physically-inspired atmospheric scattering sky shader in the examples/addons. Tunable sun position, turbidity and Rayleigh scattering produce a continuously adjustable sky, useful for a day/night cycle or a sunset that shifts in real time, something a static texture can't do without swapping assets.

Custom shader: full control for a stylized (non-physical) sky: a gradient, animated stars, a painterly look. Worth it specifically when the built-in options' physically-inspired look doesn't match your art direction.

Clouds in real-time rendering are almost always an approximation rather than true volumetric simulation, layered by cost:

• Billboard/sprite clouds: flat, camera-facing textured planes (cloud puff textures) scattered in the sky. Cheap, looks reasonable from a distance, breaks down under close inspection or when the camera moves through them.
• Layered noise on a sky dome: procedural noise (often multiple octaves scrolling at different speeds) sampled on the inside of a sky sphere or dome, for a stylized moving-cloud look without discrete billboard shapes.
• True volumetric clouds: raymarched noise through a 3D volume, the technique behind the most convincing real-time clouds (and behind most AAA game skies). More expensive; usually only worth it when clouds are a genuine focal point of the scene rather than background atmosphere.

THREE.Fog (linear, fades between a near and far distance) and THREE.FogExp2 (exponential falloff, generally looks more natural) both blend distant objects toward a fog color, which does real work beyond atmosphere: it hides the far clipping plane's hard edge, gives depth cues for free, and can mask level-of-detail pop-in at a distance.

Set scene.fog, and make sure the fog color matches (or complements) your sky/background color. Mismatched fog and sky color is one of the most common "why does my scene look wrong" issues in outdoor Three.js scenes, since the horizon line becomes visibly discontinuous.