SRD:Solid Fog

This spell functions like fog cloud, but in addition to obscuring sight, the solid fog is so thick that any creature attempting to move through it progresses at a speed of 5 feet, regardless of its normal speed, and it takes a –2 penalty on all melee attack and melee damage rolls. The vapors prevent effective ranged weapon attacks (except for magic rays and the like). A creature or object that falls into solid fog is slowed, so that each 10 feet of vapor that it passes through reduces falling damage by 1d6. A creature can’t take a 5-foot step while in solid fog.

However, unlike normal fog, only a severe wind (31+ mph) disperses these vapors, and it does so in 1 round.

Solid fog can be made permanent with a permanency spell. A permanent solid fog dispersed by wind reforms in 10 minutes.

Material Component: A pinch of dried, powdered peas combined with powdered animal hoof.