[Ranylyn]

"Reduces the FP cost of Weather Domain spells by 2."

I'm going to go out on a limb and assume you mean Nature. because Lightning Tome, and Lightning Damage is the Nature Domain. I have never seen a spell under the "Weather Domain."

Storm chapter does not reduce the FP cost of spells. Tested with a Rank 1 Rye. 9 FP wth and without the tome.