One thing I forgot to note: our defacto allies, Warbaxia, have Arcane Lore now. Govannan can't be far behind.
I've been doing some research on Blinding Light, and its odds are pretty bad. Base resistance is 50%, +10% in cities, +5% per level of unit, +20% for hero. So Gibbon would be at 95% resistance outside cities, 100% in cities. One more promo needed for 100% resistance outside cities. If that's their plan for counterattack against us, I think I'll take those odds.
For my own reference, I'm adding a list of promotions on the caster that will alter the resistance of units being attacked (Rathas/Radiant Guards don't start with any of these

):
Avatar -50
Channelling II/III -10 each
Potency -10
Metamagic I/II/III -5 each
Puppet +20
Tower of Alteration -20
Spell damage:
Combat I/II/III/IV/V +5 each
Metamagic I/II/III +5 each
Puppet -10
Unholy Taint +10
On defense, units get a base resistance of defense*2 + level*2 + (city cultural defense)/4. Any type resistance (death, fire, lightning, etc.) is added to this base number.
Nominal damage is the base damage of the spell (40 for Snowfall) plus any modifiers. Two random numbers in the range (0-nominal) are generated, and added together. This
sum is multiplied by (100-resistance)/100.
If this modified damage is greater than the spell max damage (80 for Snowfall), it is limited to the maximum.
tl;dr: two snowfalls should, on average, do the max damage. The variance on damage is very high, so there may be a few lightly damaged units.
Snowfall will turn all land tiles surrounding the caster to Snow for 3-8 turns, and has a 2% chance per tile of spawning a Blizzard. It will put out smoke or flames.