Pathfinder 1E Storm of Vengeance question

Are the effects each round sequential or additive?

Sequential would be like the storm is flipping through a deck of cards, trying to find an effect it likes. Averaging the damage, the Lighting victims will have taken less than 40pts of damage, and anyone else in the storm will have taken less than 20pts of damage.

If the effects were additive each round then the storm would be a legendary event. Averaging the damage, the lightning victims (if different people were hit each round) will have taken around 160pts of damage, and anyone else in the storm will have taken around 130pts of damage.

As a point of comparison, Implosion cast at L20 deals 200 points of damage to each of ten people over the course of the spell.

The stong point of Storm of Vengeance is the huge area; you can obliterate a small town with it.

Afraid the damage isn ot additive in my reading. However, I've never ever seen the spell used. And a large area can be just as much of a hindrance as an asset. I suppose it is rather under-powered with my reading, or at least of low utility.

The Acid Rain and the Hailstones seems like being additive would be fine, though I'd probably say you only get lightning bolts every 3rd round. That's still 180d6 worth of targeted lighting, but it gives enemies a little breathing room to counter if they're not wiped out by 4 rounds of acid rain and 2 rounds of hail for a stunning total of 14d6 to everything not protected.

But still, it's a 9th level spell that requires you to stand still for 10 rounds and make concentration checks against your own spell provided you are not otherwise protected. I tend to be fairly generous with my readings of 9th level spells because ya know, they're 9th level spells!

So...
Round 1: Thunder
Round 2: 1d6 acid rain
Round 3: 1d6 acid rain & lightning bolts
Round 4: 1d6 acid rain & 5d6 hail
Round 5: 1d6 acid rain & 5d6 hail & winds
Round 6: 1d6 acid rain & 5d6 hail & lightning bolts & winds
Round 7: 1d6 acid rain & 5d6 hail & winds
Round 8: 1d6 acid rain & 5d6 hail & winds
Round 9: 1d6 acid rain & 5d6 hail & lightning bolts & winds
Round 10: 1d6 acid rain & 5d6 hail & winds

That'd be my ruling.

Looking at it again, and considering it requires concentration, I think I'm fine with all effects being cumulative. Yes, that is a LOT of damage, but nothing stops people from leaving, and even minor resistances will make you mostly immune. At its level, its not that impressive.

With the very most generous interpretation, you could inflict an average total of 381 points of damage on a single target. But considering that the caster cannot see to target the bolts after round 4, the practical maximum is 210 points average. A modest DR and Energy Resistance of 10 would cut this down to a very non-impressive 95 points of damage. And that is assuming all saving throws fail and targets can't escape the area, and the caster can spend 10 rounds concentrating.

I can actually see how this could be used as a challenge for a mid-level party - get out of town while the evil high priest (and his army of escorts) razes it. Very Thulsa Doom!