Tornado

[galehar]

To avoid a too big difference between damage vs AC and no AC, it has been suggested on IRC to apply AC/3 per turn (so 2xAC over 6 turns).
Base damage is 6d(power)/n (currently, n=10). Let's find a good value for n. I've made spreadsheet.

15?

[Galefury]

Wow, I completely forgot about cloud damage. I don't know how you calculated the effect of AC on damage, but note that for low damage attacks (like clouds) estimating post-AC average damage by average damage - AC/2 overestimates the effect of AC (by neglecting damage roll often being smaller than AC roll). Probably not a large enough error to justify doing a real calculation though.

15 sounds about right. This would make one casting of Tornado only slightly more damaging than Firestorm + cloud damage vs 10 AC nonresistant targets, while both spells provide different tactical benefits: Firestorm does most of its damage instantly (and thus spammably) and summons popcorn and has more range, Tornado has larger AOE and you can precast it and take it with you. It would also make Tornado shine against resistant or heavily armored targets, outdamaging Firestorm by a large margin (at the cost of not being spammable and low range in cramped environments).