If the rock is wet, it's raining.
If the rock is swinging, the wind is blowing.
If the rock casts a shadow, the sun is shining.
If the rock does not cast a shadow and is not wet, the sky is cloudy.
If the rock is not visible, it is foggy.
If the rock is white, it is snowing.
If the rock is coated with ice, there is a frost.
If the ice is thick, it's a heavy frost.
If the rock is bouncing, there is an earthquake.
If the rock is under water, there is a flood.
If the rock is warm, it is sunny.
If the rock is missing, there was a tornado.
If the rock is wet and swinging violently, there is a hurricane.
If the rock has white splats on it, watch out for birds.

imparable ^^"
quelques exemples en photos: http://www.theweatherstone.co.uk/gallery.html