For too long I’ve struggled to get mist to work in Cycles the way it works in the Blender Internal render - that being mist that sits just around the horizon, giving a distant look. To my disappointment, the mist generated in Cycles tends to cover the whole sky giving an overcast look.
How does one achieve a distant horizon where mist only hovers around the horizon?
You mean the Mist generated with the Mist pass which you mix in with the compositor in post?
Simply stick a color ramp in between your Render Layers Node Mist pass and the Mix node. With the color ramp you can control wehre your mist should be
@Lumpengnom thanks so much for the reply.
i did that but i was only able to control the bottom half of the mist. the top half above the horizon still remains fully white. i want to be able to control the top half like i can control the bottom half
Is there any geometry in the top part? Mist only worjs if there is geometry present because it is basucally just a pass that colors stuff whiter with increading distance.
I had a similar problem recently to get a realistic haze layer caused by an “inversion” in the atmosphere:
Here the camera is looking above horizontal, so the haze is less pronounced as would be the case in real life. I tried a mist pass also, but had exactly the same issue you have so I made a mesh:
…which tapers at the top so I get the mist fading out as it rises from the ground, then gave it a simple volume shader:
The haze mesh is bigger than the distance from the camera to the aeroplanes, so the camera never looks through the haze, just at it in the distance. So, when you render lower to the ground and looking more or less horizontally, you get this effect:
The sky environment texture shows above the haze and the haze blurs the ground. I played with the density values and colours for the haze shader until I got the desired effect. I could have made the object a smaller section and used a higher density value, but this produced a less-than-convincing fade out at the top of the haze.
Cheers, Clock. 
PS. An “Inversion” is where the air at a higher level is slightly warmer than lower down, trapping rising air and its moisture into a haze layer that makes visibility poorer over long distances. We pilots have to contend with this a lot and it makes our airfields harder to find…
EDIT:
Just to give an idea of size, the glider has a 20m wingspan and the internal diameter of the haze is 3km.
OK - here’s yet another approach.
For this version, you have a render layer with one object in it: a sphere textured to be lighter around the equator. It has an emit material, and a copy location constraint to the camera. It gets a renderlayer all to it’s self calld MistGradwith with samples set to 1.
Then, in compositing, your main scene layer has a mist node. This gets multiplied to the colour of MistGrad, and this is your mist value.
@Lumpengnom there is no geometry, there is just a sky texture via the world setting as can be seen on the render layer input. is there a away to use a color ramp to attenuate the white on the top half of the sky that would then reveal the environment sky texture
@clockmender thank you for your detailed explination. your approach is symbolic of my frustration. to think that this is what one has to go through to blend the horizon worries me. again, i wish mist would just work in cycles like i does in blender internal render
@yogyog thanks so much. im hopeful for your suggestion but as im an advanced beginner i cant quite wrap my head around your approach. is it too much of me to ask you to maybe add some screen grabs of this node set up?
