Weather is randomly generated around Arelith and reflects to some extend location of area and time of the year. With the exception of a few mechanically represented cases, it is up to the players how much weather effects will be reflected in their roleplay.
Weather conditions will affect the state of PCs. For example hot summer makes one more thirsty.
Strong wind or deep snow may temporary slow characters. Extremely strong wind may even knock them down.
In case the decreased speed persists even in indoor areas, see the description in common bugs.
There is a fairly low chance to get hit by a lightning in bad weather. The amount of damage character take is approximately proportional to how near the middle of the bolt is character located.
The Underdark has no weather, per se, but is instead victim to a short group of phenomena, natural and otherwise. These effects tend to be specific to certain areas, with some effects happening with more frequency than others, or not happening at all.
Quakes may be averted altogether, as players will be notified through the combat log that a earthquake may be forming in the area, albeit one may be unlucky enough to walk into a quake-in-progress area.
During the quake, area-wide tremors will happen, more or less two times per minute. A reflex save will come with the tremor; characters who pass the save will be unaffected by the tremor, but character who fail the save will be knocked down for a round and take some physical damage.
When a spore storm hits an area, it will be subject to random sudden spore bursts. A character caught in the area of the burst will take some magical damage, and subjected to a fortitude save against disease.
When the Faerzress radiation strikes, all spellcasting will have a high chance of causing a random secondary effect that may be benign of negative, regardless of the intended target. Spellcasting from items that replicate spells, scrolls, wands, potions and other sources also apply.
Wild Mages's wild surges are very similar but not identical to the Faerzress mishaps.