The difference between 2 and 3 damage is enormous.
Elements has 96 creatures
Rain of Fire can one-shot 74 of them (77%)
Thunderstorm can one-shot 50 of them (52%)
Rain of Fire can kill
48% more creatures than Thunderstorm.
List of creatures RoF can oneshot but Thund can't:
Killing 77% of the game's creatures for 5 quanta seems pretty strong to me.
Especially because killing sub-3hp creatures isn't game changing most times (cept vs fire) but there are plenty of 3hp creatures that can ruin your game if not killed: Crim Dragons, Forest/Fire Spectres, Mummies, Gargs, Otys, Vamps, Shriekers, Wyrms, Wardens...
If it costed 3-4 everyone would pick 3-6 in any deck; even speedbows could use them which is pretty OP for obvious reasons.
Also, comparisons with
cards is useless since Air is based on being cheap, fire is not, if anything it's the opposite: the more quanta you have the stronger you are (Fire Bolt, Farenheit).