It's been debated
http://paizo.com/paizo/messageboards/paizoPublishing/pathfinder/pathfinderRPG/rules/archives/stormOfVengeanceEffectsAdditive
http://www.enworld.org/forum/d-d-legacy-discussion/42729-question-about-storm-vengeance.html
I've been under the impression that each effect happens just once. (Which would mean it only deals, in full sum 16d6 damage or about 48 damage over the course of 4 rounds and it must be concentrated on). It always struck me as an awfully weak spell for its level, outside of the first round deafening effect assuming you were going to surprise a large group). The rains take an awfully long time to become a factor and nothing keeps targets within its area before the rains speed reduction, albeit not small.
Damage (sans six 10d6 lightning bolts) will deal 1d6 acid rain and 5d6 bludgeoning hail. If you were to let the acid extend all 9 rounds and hail for 6 rounds that would be 6d6 per round at most (about as much as a 6th caster lvl Fireball) over a large area each round. Still not much, but it could add if allowed to run the full course ten rounds of 1d6 acid and six rounds of 5d6 hail for 9d6 + 30d6 or somewhere around 117 dmg or so. Mind you though, it's spread out over 10 rounds. A MUCH more typical 5 round situation would see 4d6 acid + 10d6 lightning bolts + 5d6 hail or 19d6 -about 57 ish damage. Really not amazing since you have to spend the whole fight concentrating. Albeit another spell like Sonorous Hum can relieve you of having to take the action to concentrate, its another spell used, and you still have to roll concentration checks when you take damage.
- GRANTED: None of this continuous damage if allowed would allow any save. Of course, the PCs probably wouldnt want to fight in the area though and Acid may be Energy Resisted, while bludgeoning could be DR'd. Plus, many fights wont last long enough to see all the damage -although nonmoving structures certainly would.
- Compare it to Meteor Swarm which shoots 4 meteors that deal 2d6 bludgeoning and 6d6 fire. You can aim it an an area or try to use a ranged touch attack on a creature. A creature struck directly takes the bludgeoning (no save) AND gets no save against the 40ft explosion of fire damage (though creatures around them do). Since you can target a single creature with more than one meteor, all 4 meteors pile up to 8d6 bludgeoning and 24d6 fire or about 96 damage. It has about 8d6 less than the above interpretation of storm of vengeance. But, much the same Resistance applies to the energy damage and bludgeoning damage can be DR'd away. Both spells allow SR.
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