When picking up any block under a tree and looking up, while its raining (thunderstorm or regular rain),

rain will render above the following blocks:

All leaf types

Stained glass blocks

Stained glass panes

Regular glass blocks and glass panes are difficult to tell. I think this is happening to them too

Barrier blocks 

Cobwebs

Trapdoors (open and closed)

Grass blocks

Dirt blocks

And the list might still go on for awhile. 

Maybe this is happening for all blocks? I'm not sure. ((I'm not going to test every single block))