|This article is about an older version of DF.|
Cave adaptation is a condition that happens to your dwarves after spending too much time underground. Every frame spent in a Dark tile increases its "cave adaptation" counter by 1, to a maximum of 800,000 (just short of 2 years), though there is no way to tell if a dwarf has cave adaptation simply by looking at their various stats.
When a dwarf with cave adaptation goes outside while the sun is out (i.e. when it's not raining or snowing), the dwarf will gain one of two negative thoughts. If the dwarf has mild cave adaptation (at least 1 year spent in the dark), he will gain the thought 'irritated by the sun recently' and suffer a small amount of pain and fatigue and become stunned for a moment. A dwarf with serious cave adaptation (at least 1.5 years) will gain the thought 'nauseated by the sun recently' and will vomit all over the ground in addition to suffering greater pain and fatigue and being stunned for a while longer. This leaves a dwarf unable to defend himself, which can be a problem during combat.
A dwarf with the description, 'likes working outdoors and grumbles only mildly at inclement weather' is no more resistant to cave adaptation than a regular dwarf is. In fact, said dwarf may even have cave adaptation - this description simply indicates that the dwarf gets reduced happiness penalties from having to do without the comforts of civilization.
Fortunately, cave adaptation is not a serious issue. The unhappy thought from serious cave adaptation is strong (strong enough to totally negate the "slept in a room like a personal palace" happy thought), and can easily push already unhappy dwarves over the edge, but it is very controllable. Setting up a meeting area outside and above ground with items that cause powerful happy thoughts helps: very valuable statues are the best bet here. Be sure to wall it in or otherwise protect it so the area is not a liability during a siege (and don't forget flying creatures). Cave-adapted dwarves will come out, admire the items in the meeting area, get a happy thought, then vomit all over themselves. If you do it right, you roughly break even. Prolonged, regular exposure to the outdoors (even if it's raining) WILL cure cave adaptation - spending one day outdoors negates 10 days spent in the dark. Dwarves who vomit all the time don't seem to get hungry any faster.
If your trade depot is outside, cave adaptation can create the amusing scene of thirty dwarves carrying goods to the depot all emerging and throwing up at once. Fortunately, the merchants seem to take this in stride, and it doesn't seem to negatively affect negotiations.
 Preventing and curing cave adaptation
The easiest way to deal with cave adaptation is not to go outside at all. On most maps this is quite manageable, e.g. with traders bringing enough wood, making bolts from your own cattle (no hunting), et cetera.
Curing and prevention of cave adaptation are achieved through the same means: regular, extended exposure to the outdoors, sunny or not. Indoor light will not treat cave adaptation, nor will it trigger the ill effects. An outdoor area to fight cave adaptation may be accomplished by having a meeting area or statue garden exposed to sunlight, with no roof. An armor stand or weapon rack placed outside in the same manner and made into a barracks for sparring will prevent cave adaptation in your military and is highly recommended, as constantly vomiting champions aren't particularly useful during combat.
If you build your fortress in entirely above-ground areas, your dwarves will never become cave-adapted in the first place - if you start them off underground and then move them to the surface, they will vomit as they get used to the sunlight, but living in constructed surface buildings (or even a cast obsidian tower) will not cause them to relapse.
A dwarf who has spent his entire life in the dark will need to spend about 2 and a half months outside in order to be fully cured. Occasional or brief exposures to sunlight cannot prevent cave adaptation, though it will still make them suffer; negative thoughts cannot be prevented in those who suffer cave adaptation, but can be treated with a really, really awesome dining room.
If all else fails, simply give up on the outside world and have your dwarves spend the rest of your fortress's life lurking in the bottom of a dark cave gnawing on a cold plump helmet.
You can also cure cave adaptation by cheating, and just removing the tag from the dwarves' RAW files. Under "creature_standard.txt", remove the [CAVE_ADAPT] line and save.
 Proper Perspective
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Dwarven philosophers have pointed out that this odd human labeling misconceives an advantage for a problem and that in fact "sun adaptation" is rather something to worry about. Blinding and losing control over bowel movements is nothing but a strong and appropriate warning signal from exposing the so-amazingly-adapted dwarven eye to the piercing sunlight. More so, since there is no good reason for exposing oneself to the various uncontrollable surface dangers in the first place. Does not the best wood come from tower caps? How can one spend "too much" time underground? How is the dwarven adaptation to its natural environment (which is, by the way, a birth gift, not acquired) objectionable? Is it the human eye that picks out the greatest gems from inconspicuous rock? Or, in the words of Zuglares: "Does even a human look into the sun?" Did not the great poet Uristophanes say "Is there ore? Is there gem? Is there proper rock? Does the purple one spread its shield? What are you longing for, clouded mind?"
Pragmatically put, dwarves belong inside, buildings belong inside, if it means showing traders our great halls, all the better. If one lusts after strange surface crops, there are other ways. A true dwarf strives for his own tree farm. The hunter's fate is that of a pitiful outcast, sacrificing his sanity to serve his home. Who misses the petty glowball when one can admire legendary halls and artwork? Give the surface what it deserves - your refuse.