Rain Wash  

Rain washes away blood and dirt. This includes the character's body, clothing worn, bags equipped, weapons equipped and attached, the vehicle your character is in and the vehicle attached to it, or the vehicle most recently used and the vehicle attached to it.

How it works
Character, clothes, bags, weapons
When it's raining and your character is outside (but not in a car), the things mentioned will slowly clear. The process will begin as soon as the part of the body* or the object becomes wet: some of the blood will turn into dirt, and then the dirt will be washed away.

Not everything is cleared at the same time! Notice how the humidity increases: top layer/uncovered body parts -> inner layer -> covered body parts. It's vanilla mechanics.

The cleaning speed of the object or body depends on the intensity of the rain and the humidity of the object or body. Not all dirt will be washed off clothes! Depending on the waterproofness of the garment, up to 20% of dirt will remain. Not all dirt will be washed off your character! Up to 10% dirt will remain.

Transport
The vehicle your character is in and the vehicle attached to it will be cleared when it rains.

The last used vehicle and the vehicle attached to it will also be cleared after your character exits it. This works as long as your character is about 100 tiles away from those vehicles. After a reboot, you need to log in/out again for this to work.

Wash in the rain
When your character is outside and it's raining, right-click and you'll see the option to wash in the rain. Works like "Wash" but slower and your character will take off all the clothes (and put them back on when done).

Settings
If you want to customize the mod, you need to install and enable Mod Options.
Next Settings -> Mods -> Rain Wash.