some of the weather effects are driven by “in a cave (approximately, based on original world state)” and "in sunlight (direct or indirect, game doesnt know the difference (*)), and some weather particle effects collide with a heightmap of the world (most rain/snow particulates).
rain/snow particles that collide with the terrain will be stopped by most blocks from above (but square/bevel/trellis chiselled blocks are too detailed to work with the system and generally just allow the particles through).
the snow-covering effect is driven by “direct” sunlight (*) as determined by whether the maximum amount of sunlight that reached a position during the flood-fill had to pass “through” a block (foliage/glass/panes) on the way there.
things like the “fog” of rain systems is simply driven by (indirect) sunlight, so as long as you arent “in a cave” then you will get the fog even if your house has a roof on it, if your room is full of sunlight…
its possible we would be able to use the “direct” sunlight that the snow covering uses to also modulate these effects, which would improve behaviour so that as long as your room is “sealed” (no air-gaps) then the sunlight wont be “direct” and so will prevent them appearing the way the snow-covering effect would be prevented.