Maya 2012 Tutorial
How to Add Volumetric Fog in Maya
One way to create fog is to use a Volume Primitive. As you add the primitive, it is automatically connected to a Fog node. Once connected, you can use the Fog node to fine tune the effect.
Add a sphere volume object to the workspace by choosing Create > Volume Primitives > Sphere.
In the Attribute Editor, fog tab, click the Color rectangle to change the fog color.
To select a gradient color, open the Ramp section and enable the Color Ramp Input.
Click the IPR icon, then marquee drag in the preview window to view your adjustments in real time.
Adjust the gradient values and note the changes in your render window.
Density and Edge Dropoff can be set in the fog tab.
As objects pass through the fog, they are affected by their depth inside the sphere.