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.