Weather is not a concern underwater, or in caves.

Battles typically take place under clear skies. However, it might be Raining when a battle occurs, or Snowing instead if the Temperature is Cold.

Rain only happens in provinces with a temperature of 0 or higher; or, in other words, provinces that aren't cold. It only affects outdoor battles, of course. The chance of Rain is 10%, plus 10% per point of Growth (minus 10% per point of Death). Middle C'tis's Dominion also causes Rain.

Rain is unfortunate weather for Fire Mages, though it used to be a lot worse for them. Spellcasting Fatigue for fire spells that don't work underwater is increased by 50%. Rain also swiftly puts out burning units.

Other side effects of Rain include clouds dissipating 25% faster, and the act of Flying being twice as Fatiguing; the initial cost of Flying is equal to your Melee Encumbrance.

Snow only happens in provinces with a temperature of 1 or lower; or, in other words, provinces that aren't warm. It only affects outdoor battles, of course. The chance of Snow is 15% per point of Cold; Growth does not increase the chances.

Snow is slightly better for Fire Mages than Rain, in that there isn't a special Fatigue penalty just for their spells; otherwise, it's just like Rain. It might also not dissipate Frost Clouds, such as those from a Chill Aura, though it still dissipates other Clouds.

Many Battlefield Enchantments force weather, changing it until the caster leaves the field (or gets obliterated). Rain is the simplest one, an Evocation that lets 3 fellows with 1 (or 2's with 2) knock water out of the sky, forcing whichever wet weather is allowed by the temperature. Stygian Rains and Storm do the same, though with different intentions. Blizzard, a 43 variant of Storm, forces Snow by setting the local province to 1 for the duration.