How's about this. Just made it up. Adjust the spell level as you see fit.
Divine Rain
Transmutation/Conjuration [See text]
Level: Clr 5, Drd 6, Pal 4
Components: V, S, M, DF
Casting Time: 1 round
Range: Long (400 feet + 40 feet per level)
Effect: One 100 foot radius cloud that rains holy or unholy water
Duration: 1 round / level
Saving Throw: No
Spell Resistance: No
Upon completion of this spell, the caster points upward to the sky, and a thin gray beam shoots from his finger. By the end of the caster’s turn, a cylindrical cloud forms directly above him that rains down either holy water or unholy water, depending on the version of the spell cast. This cloud moves with the caster, keeping the center point of the cloud above the caster’s head at all times. This rain inflicts 2d6 points of damage on any creature within the radius of the cloud, if that creature would normally be harmed by holy or unholy water accordingly. If the sky overhead is already cloudy, then a section of that cloud is affected by the spell instead.
If the holy water version of the spell is cast, this spell contains the Good descriptor. If the unholy water version of the spell is cast, the spell contains the Evil descriptor.
Material Component: 5 pounds of powdered silver (worth 25 gp).
I know that (un)holy water normally does 2d4 damage, but I upped it because a flask of holy water is a single flask, while being rained on is fairly constant.
EDIT: Upped Cleric and Druid Levels by 1.