Rain

Rain is a feature than can be activated by pressing F5 ingame (not in indev). It has no effect on the gameplay yet. Rain will only fall where the sky isn't blocked, so on blocks which are also receiving sunlight.

In Indev, Rain is replaced with 3rd Person.

Invisible Rain glitch
In Infdev pressing F5 will not only cause you to go into 3rd person but also create invisible rain. The only evidence of there being rain is particles being created around you.