Minecraft wiki entry for Tick, associated text: Mobs naturally spawn.
During a thunderstorm, lightning may strike somewhere in the chunk (1⁄100000 chance).
1⁄16 chance that one column is chosen for weather checks on the topmost block:
If in a cold biome, water freezes into ice if possible.
If snowing, a snow layer is placed if possible.
Additionally, cauldrons can be filled with powder snow.
If raining, a cauldron is filled with water.
A certain number of blocks within the chunk receive random block ticks, as described below.