Some experimentation with unhealthy temperatures established some new knowledge:
- dwarfs can survive outdoors for a while in climates that flash-boil entire lakes. After a few days, they'll start to melt, though. If you abandon the fort before they die, they can still immigrate to new forts with nothing worse than missing head fat. It was quite funny to look at the lake and river that kept spawning clouds of steam at the map borders. Brooks will also boil away - steam forms underneath the "brook/dry brook" surface.
- in ultra-hot biomes, your embark party cannot set foot on the ground. They spawn on the map border, unable to move until they die of blood loss.
- i haven't found a biome too cold to properly embark on: the embark party has always been placed properly. In deadly cold biomes, all plant and animal materials will decay almost instantly. This means your wagon and all your food. Booze will keep sitting on the floor as a barrel-less glob, i've not found a way to make that useful. My embark equipment was quickly reduced to two picks, two axes and an anvil. Staying outside for long enough will eventually kill animals and dwarfs, dwarfs may last a bit longer because their clothes protect them for a while, but may decay and disappear if it's cold enough. All my starting animals and two migrants died of cold. Another migrant survived but suffered frostbite in all of its
teeth.
Fortunately, the caverns were very shallow, so with a bit of judicious herbalism, i could kick-start food production in spite of a short scare when one miner got hungry enough to try hunting vermin. ("Hunting vermin" is the kiss of death for a fledgling fortress - it will override any other job and will be performed regardless of whether there's any huntable vermin around; your dwarfs will starve to death because they unsuccessfully hunt inexistant vermin instead of doing something to actually get hold of food.)
I only get dwarven caravans, and the cold destroys most of their wares on the way to the depot.
- I don't know what the cut-off points are, but in very hot and cold biomes, no wandering critters seem to spawn. I have a scorching desert biome that hasn't seen a single non-civ visitor aboveground, and no animals have spawned in the ultra-cold terrifying mountains. Underground creatures do spawn, and caravans as well as thieves show up, too.