Quicksand as raindetector

Quicksand could have a slight texture difference than sand and comes with extra features. You'd find it in biomes such as swamps and rivers.

Function: when dry, it slows movement (same as soulsand). When wet, it sinks players and mobs by one block (except for mobs such as spiders, cats and frogs). Quicksand usually is not deep enough to drown in. Jumping and movement increases sinking speed. Crouch and move to escape the quicksand, but movement would be severely decreased.

Redstone feature: observers would detect the state change when wet or dry.