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. Instead of rain during the Winter, it will snow.

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 not strike a player directly (unless playing as WX-78); however, it may 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 the surroundings 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 the player slightly colder and does not damage WX-78 the same way rain does.

Frog Rain
During Spring in Sandbox Mode and the "A Cold Reception" and sometimes the "The Game Is Afoot" chapters 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. Frogs produced from rain will stay on the field until they're killed, and while they don't sleep at night, they will stand in place.

A good strategy to get rid of frogs and get an easy loot is to quickly move near to Beefalo herds right after frog rain starts. Game will automatically drop some of the frogs into the herd. As beefalos always be in heat in Spring, they will attack to frogs which will start a frog - beefalo war that will end with an easy beefalo win. In order to grab the Frog Legs from the ground right after frog rain, make sure to bring a Beefalo Hat since if player wears a beefalo hat, beefalos will not attack the player when they are in heat. This strategy provides the player to get almost two stacks of frog legs.

Reign of Giants icon.png of Giants
In Reign of Giants DLC, rain causes wetness to the player, which makes items less useful and greatly damages sanity, especially if the player wears wet clothes. Also, the chance of rain will change depending on the season. Although it may seem like a minor change to the game, this new mechanic has caused starting the game in Spring to be difficult (especially while using WX-78), due to freezing and major sanity loss.

There is also a strong possibility of getting struck by lightning if there aren't any objects (trees, grass or any burnable item) or creatures in the vicinity, which will deal 10-15 damage to the player.

Frog Rain also occurs in Sandbox Mode in the Reign of Giants DLC, though only the during the Spring season. The number of Frogs spawned per rain will increase as the days go by.

Trivia

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