Weather refers to any type of weather effect in DF. It includes snow, rain, and special features in evil surroundings.
Weather can be disabled by changing [WEATHER:YES] to [WEATHER:NO] in the d_init.txt file. Disabling weather is a quick and largely harmless fix to improve framerate on older machines if required.
 Normal Weather
It is raining.
When rain hits a tile labeled as a murky pool, it will begin to fill it up with 1/7 water, and if that does not evaporate the water will grow deeper, until the pool is full. Murky pools do not overflow from rain, but this extra water can be drained off and stored or used. (See the Well Guide.) While not much, it can really help maps without 'unlimited' water supplies such as rivers and brooks.
 Snow and Cold
Winter is here. A snow storm has come.
When water comes into contact with a freezing climate, such as winter months in a moderate biome, or at any time in a tundra or glacier, it will freeze into a wall of ice. Freezing ice acts much like obsidian, and will instantly kill anything caught inside of it -- including creatures otherwise extremely hard to kill. In moderate biomes after winter has passed, walls of ice will melt back into ponds. No matter what the height of the water was before freezing, it will always thaw back into a full 7 units of liquid.
Dwarfs outside during a snowstorm can also freeze to death, so a very high priority when embarking on a glacier is to dig out some place warm for your idlers to rest.
 Evil weather
Certain evil surroundings feature freakish weather, such as fogs, clouds, and rains. They may afflict those caught in them with various kinds of syndromes or curses, such as poisonings or transformation into zombies. Names for evil weather are randomly generated, typically something along the lines of "abominable mist", "unholy gloom" (clouds) or "creeping murk", "horrid goo" (rain).
Two kinds of evil weather exist - evil rain and evil clouds. The type of weather effect and their associated syndromes (if any) are different and randomly chosen for every evil biome in a generated world. Usually there will be only one weather effect for a given evil biome, often in conjunction with the effect of corpse animation.
 Evil Clouds
A cloud of haunting fog has drifted nearby!
Evil clouds are made of a generated inorganic gas or dust. They start in one tile and spread out in a similar manner to miasma vapor; when the game announces the cloud's presence, it will zoom into this tile. Evil clouds cause more serious syndromes than evil rains, similar to those of forgotten beasts, titans and demons. Certain evil clouds transform living beings caught in them into dangerous zombie-like thralls, turning them against all life while significantly increasing their strength and toughness attributes.
Evil clouds inflict worse syndromes than evil rains; thralling clouds are especially dangerous in the extreme, as the zombies produced are much stronger than those produced via ambient effects. These undead are very hard to kill and are much stronger than the unit they inflicted. Evil clouds have a tendency to roll over your outdoor trade depot and convert traveling merchants into a band of dwarf-hungry savages, which can be crippling to a fortress reliant on trade.
Thralling clouds are also dangerous in adventure mode. Undead status means hostility from civilized beings, reduced speed and no regeneration. However, other undead will ignore you.
 Evil Rain
Evil rain can be made of either the blood of a civilized race (dwarves, goblins, etc.) or a randomly generated inorganic substance. Blood rain typically causes no syndromes, only giving whomever caught in it an unhappy thought. Generated substances are more dangerous, causing minor symptoms such as vomiting, blisters and fevers, as well as inspiring the aforementioned unhappy thought. Evil rain can also cause wounds and injure dwarves and other creatures caught in them, although the extent of injury depends on the substance. Dwarves spattered with evil rain will seek a bath after any outdoor work, wasting time and soap. There are rumors of rain composed of other substances, such as alcohol; however, this seems to be rare.
 Dangers of evil weather
One of the dangers of evil weather is that ponds will never refill. Store water into underground basins and plan accordingly. Another annoyance is the total lack of wild bees in an evil climate. Though evil rains (excluding rains of blood) usually cause the "milder" types of syndromes, these may still cause death as a secondary result of that syndrome: e.g. suffocation from blisters, dehydration from chronic nausea.
Some evil weather effects condense on creatures, making their effects semi-permanent - symptoms won't go away until the source is washed. Those effects also can spread like a disease. In most cases, it will just infect dwarves that carry contaminated bodies to caskets. Thralling dust clouds, however, can quickly lead to an unstoppable zombie apocalypse if the dust is not completely washed off somehow.
 Movement of evil weather and safeguarding
Evil weather forms on the edge of the map and moves about the screen in one direction as the wind blows. Because the clouds inflate similarly to miasma, they can very quickly dissipate, fly out of the map immediately, carry on at a steady pace, or spread rapidly engulfing very large areas. They will not, however, change direction.
Although it will glide along a single Z-level, it will climb straight walls and pass over structures. It will not move through a closed door or raised drawbridge, but it can descend from an open roof and it will pass through fortifications with ease. If you choose to fortify outside, be sure to have any open areas secured with floodgates to act as shutters to keep the clouds out. Adding doors throughout the structure to stop any accidentally contained clouds from moving further inward does not hurt.
Undead created by evil weather gain immediate strength bonuses and acquire a status of Opposed to Life. They will tirelessly seek out living things to kill until they are struck down, which can take considerable effort.