"... simulates atmospheric conditions such as haze and fog in your composition. Layers further away from the camera appear to be more deeply immersed in the haze/fog than layers closer to the camera. This also applies to parts of layers: If one edge or area of a layer is closer to the camera, this area will appear less immersed in the haze/fog than the pixels further away."
Here's a tutorial from Digieffects:
Atmospheric effects without a filter assist can be complicated; here are some resources:
- a free excerpt from the book After Effects Studio Techniques by Mark Christiansen, Climate: Air, Water, Smoke, Clouds in After Effects 7.0
- AEP 's Fractal Noise: fades & more
- AEP's Creating fog in After Effects
- a few implementations in tutorials by Video Colpilot, for example Smoke Screen, Stabilize Shaky Footage