Changes in mob behaviour can let you know rain is about to come down

Most of the info here is based off of real life.

What happens at the moment:

1. Bees enter their hives and villagers enter their houses but only once the rain has already started

What can be changed currently

1. Passive mobs such as cows, pigs, sheep etc. find shade and/or sitdown

2. Bees return to their hives and villagers enter their houses before the rain

3. A halo appears around the sun or moon 

4. Different cloud pattern, (altocumulus clouds) and/or a slight change in the sky's colour

What can change in the future

1. When frogs are added they will sound louder

2. Ostriches put their heads in the ground (obviously not real but kinda funny)

3. Vultures stop flying and sitdown on trees

None of these seems too extreme so I think it would be a pretty neat addition to the game