Every tile in Dwarf Fortress is described as Outside or Inside, Light or Dark, and Above Ground or Subterranean. These keywords can be observed from the menu, when the cursor is located over any single tile. Currently, only the 3 following combinations are possible:
- Outside Light Above Ground
- Inside Light Above Ground
- Inside Dark Subterranean
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 open and closed 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.
 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.
Ice will allow light to pass through, marking tiles as "light" and "above ground."