Depending on the Difficulty that has been selected at the beginning of the game the chance and duration of weather effects are different.

Weather changes appear twice a day and effect depend on the mode.

Easy 25% of bad weather lasting 1/6th of a day and effects on trolls are halfed.
Normal 50% of bad weather lasting 1/3rd of a day and effects on trolls normal.
Hard 75% of bad weather lasting 1/2lf of a day and effects on trolls are doubled.

There are four types of bad weather, only Rain, Storm and Snow occur naturally. Longest winter can be summoned at the Totem.

Rain Storm Snow Longest Winter
Trolls Trolls Trolls All effects of
- heat / 3 seconds - health / 3 seconds - heat / 3 seconds Rain
wet effect Storm
Plants Plants Plants
+ mana/ 3 seconds - health / 3 seconds - mana/ 3 seconds
Chance to spawn Herb
Lightning winter sleep

The chances of the weather types to occur are equal.
Weather can be influenced by praying to the gods, but bad weather cannot be changed until it has passed.