[Anaraxes]

Oh, the mention of stacking Area spells reminds me: there's a Weather spell called "Spark Storm" that you might use as a model for making a Rain of Fire variant that drops Fireballs. Spark Storm is a Windstorm that also drops a lightning bolt at random, one per turn. So it might serve as a balance comparable for a Rain of Fire that also dropped one Fireball per turn.

(Not that I was ever a big fan of Spark Storm. Too expensive for the effect, so it loses in the action marketplace to the competitors for your multiple turns of Concentrate and FP.

And ironically, the bigger you make it, the less effective it as, as it's exactly one bolt per turn in the entire area. Imagine a Spark Storm the size of the Earth! Awesome, right? Well, that adds one lightning bolt per second to the 100 or so that normally hit the Earth every second, round the clock. No one will notice except a very careful statistician... better stick to the two orcs in the 10' room.)