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Every tile in Dwarf Fortress is described as Outside or Inside, Light or Dark, and Above Ground or Subterranean. These attributes can be observed from the menu, when the cursor is located over any single tile.
There are 8 different combinations, and only 4 of them occur naturally:
- Outside Light Above Ground - Outdoors, or within outdoor-like conditions (such as being indoors, but with no roof above the tile, providing direct access to sunlight.)
- Outside Light Subterranean - Impossible
- Outside Dark Above Ground - Impossible
- Outside Dark Subterranean - Impossible
- Inside Light Above Ground - Constructed buildings on the surface, below-ground-level chambers with constructed floors or buildings only (possibly walls alsoVerify) above them
- Inside Light Subterranean - Impossible
- Inside Dark Above Ground - Possible in arena, does not occur in Adventure mode. It can occur in Fortress mode, e.g: after unretire on stockpile tiles (biome becoming embark tile level biome).
- Inside Dark Subterranean - Underground tunnels, including most of your fortress
Being in the dark increases cave adaptation, and being outside causes it to decrease (along with stunning/nausea if the sun is out).
The entire fortress site starts out as Inside Dark Subterranean. After that, a ray for Outside and a ray for Light Above Ground falls straight down from the highest z-level for every (x,y) position on the local map, changing each tile's modifiers until something that blocks that particular "ray" is encountered. Due to this behavior, any of the previous three tile combinations will never be encountered in the same (x,y) position on a lower z-level than a higher numbered combination.
 Outside vs. Inside
This attribute is determined by whether there is a roof of some kind over a tile. Weather only affects tiles which are Outside. Almost any kind of geometry will block the Outside ray including natural and constructed walls, floors, stairs, and fortifications, and bridges, floor grates, floor bars, and hatches. A tile which is Outside will always be Light Above Ground, but an Inside tile may be either Light Above Ground or Dark Subterranean.
Tiles under lever-opened bridge, hatch, grate or floor bars will keep counting as inside until any tile in that 1x1 column is built, constructed or dug on to force an update; remaining so after closing until another update.
 Light vs. Dark and Above Ground vs. Subterranean
Currently these two attributes are determined in exactly the same way. Once an area is exposed to the outside world, it is Above Ground and Light (and thus channeling this tile will make the tile below it also Above Ground and Light). Even after being covered up again, it will remain this way. This allows growing outdoor crops inside safely.