See Helpless.

Melee attacks on targets in the cloud are not possible without immediately exposing oneself to the cloud, unless the weapon being used is long enough to reach through the entire cloud. For example, a 8’ long polearm could strike a medium-sized target in a 10’ deep cloud. Note that when judging these scenarios, since we don’t use precise positioning in melee, the target should be assumed to be located at the farthest possible point in the cloud from the attacker, to give the defender the benefit. The attacking weapon must be able to reach this farthest point.