Rain

Rain is a type of weather that occurs somewhat randomly during summer. It has various effects on the game world and on the player's. The chance of Rain can be predicted by building a Rainometer, much like the Winterometer measures temperature.

Negative Effects
The character's  will slowly decrease by up to 3.3 points (depending on precipitation rate) every minute when it is raining. This can be avoided by equipping an Umbrella. Rain also reduces the efficiency of Torches, Campfires, and Fire Pits, causing them to burn out more quickly. WX-78 will take damage from rain.

If the rain is especially strong, lightning strikes may occur. Lightning will rarely hit a player (Not sure of damage output if any.) It may also strike any nearby object that can catch fire, such as Trees, Saplings, burnable structures, or even creatures. If structures or plants are too close together, such as in a Berry farm, lightning can quickly cause a devastating fire. Effects caused by lightning can easily be avoided by crafting a Lightning Rod.

In rain, fuel burns faster (depending on precipitation rate):
 * Fire Pit up to 200% faster
 * Campfire up to 250% faster
 * Torch up to 150% faster

Positive Effects
In rain, Farm crops grow up to 300% faster (depending on precipitation rate). Additionally, Mushrooms that have already been picked will only regrow after it rains.

Snow
During Winter, rain is often replaced by snow. It will cover all surfaces with a layer of snow, and make it harder to see when it is falling. Snow will all melt away when it turns to Summer, or if it rains during the Winter.

Setting rain to lots (infinite) on world presets will also set infinite snow during winter.

Snow makes you slightly colder, and does not damage WX-78 the same way rain does.

Frog Rain
During the "A Cold Reception" chapter in Adventure Mode  Frogs will rain from the sky. This is very dangerous because frogs attack the player when they are too close. It is advised to not wander too much during this type of rain.

Frog Rain will also occur in Sandbox Mode in the upcoming Reign of Giants DLC, most likely during the Spring season.

Reign of Giants icon.png Reign of Giants
In RoG, rain causes wetness to rise. Also, the chance of rain will change depending on the season.

Lightning strikes may also hit a player with a high chance.

Trivia

 * Rain was added in the A Little Rain Must Fall update.  Snow, another form of precipitation, came in the prior update, Winter.