How to make a good fog effect?

I am currently working on an rpg type game, and while in a somewhat large city, it is very laggy. I am in the unity program, not on a network, just me and myself. I thought that the problem was maybe there were too many polygons on the screen at once, so i reduced the overall view, but now i need to make a good fog effect so that buildings dont just appear out of nowhere. Does anybody know how to make a good fog effect, or know a way around the lag i am experiencing? thanks

You can turn on fog in the render settings, or you can manually add fog into your shaders.

Take a look at this answer Creating Mood and Atmosphere with lighting in Unity?.

There is a lot of information about lighting, fog, aerial perspective, colors etc.

i made the fog in the render settings to the color i wanted it (my case was grey). i then went to the camera and changed the camera background to the same/near the same color as what i made the fog and i then made a particles effect to go with the fog so it made it almost give it a Silent Hill kind of feeling with the ashes that are falling. it made the fog look good on my part give it a shot.

Method 1 - Simple and easy:
Window → Lighting
Scroll down and check fog
Edit settings to your liking

Method 2 - Slightly faster, more customization:
Remove Global Light
Add a point light, child to player
Create a material for skybox color
Window → Lighting
Set skybox material to newly created material
Make sure Sun says None
Set Ambient Source to color
Set the ambient source color
Edit the point light, skybox color material, and ambient source color to suit your needs

Hello. My friend this is how i created fog effect. Create empty object add particle system and use these settings. NO lag and pretty good effect. I used this for my 2D game. Just rotate and you can use it for 3D.