As of Alpha 14.5 rain is a precipitation system that can fall in any of the worlds Biomes. It can dampen the player and be beneficial if the player is overheating. However, if the player is wearing no clothes or armour, rain can cool the player down rapidly and cause the Hypothermia debuff. Wetness is ONLY affected by the coat and hat slot.
Rain can also reduce a measure of heat produced by a player, reducing the chances a screamer will detect the heat and come to investigate. If there are multiple sources of heat being used though, it may still be detectable.
The player has a level of saturation that is displayed as a percentage. The more layers the player has on the less the rain can affect them.
Sometimes thunder can be heard during a rainstorm.
Snow like rain falls at random times during the game, however, the criteria for which precipitation type to fall is unknown. It is assumed that Biome ambient temperature is a factor into whether snow or rain falls.
Fog can be observed in any Biome including and can vary from a slight haze to a dense cloud vastly reducing the players visibility.
Cools down the player.