How do i make a simple fog effect like this one

Hello,
how do i achieve that kind of fog effect so the objects are fading out when they come nearer to the bottom?
Is it even a fog effect or some kind of gradient color material? Unfortunately i did not find any samples or hint that could help me out.

Would be greatful for any help.

 

both are ways to do it. The gradient in the shader could be done using the worldposition.y to blend in the fog color, which is probably a bit easier than the post-effect fog version, if you want to do that by yourself (there a most certainly fog-effects/shaders that do this available for free somewehere though).