A lot of monsters have lightning resistance as well.
The difference is about immunities : most light immuned monsters have < 117% light res, so you can damage them with level 12 Conviction (-17% on immuned).

On a fireball/frozen orb build, I wouldn't bother with Thunderstorm, but be ready to use Static field.

RenTed's suggestion is good.
Static field, however, is not optional if you don't access to Infinity.
I would also leave Meteor away and increase Cold mastery until you have -150% enemy cold res.

All this except i wouldnt put that many points in cold mastery, tal orb (or was it set boni??) does give you -res too iirc. Id aim for -75 to -100 combined res (cold mastery+items) on a hybrid

Tal set provides +% cold damge, and not -% enemy cold res.

But I was precisely talking about combined -% enemy cold res.
If you stay at -75% (for say), your effective frozen orb bonus against a random 50% cold res monter is "only" +25%.
If you allocate 1 more point into cold mastery, it becomes +30%, hence a relative gain of 4%.

The same point put into Meteor will not increase your fire damage by 4% :
at best +3.7% (for the first point invested) and at worst +2.2% (for the last point).

If you put that point into Fire mastery (assuming only +7 skills from full Tal set) :
+3.9% damage for the second point invested
+2.3% damage for the last point invested
As your total +skill increases, those bonus decrease.

Actually, I don't invest anything into Meteor for a new ladder sorceress, considering there are some wasted points to unlock Meteor.
I prefer to max cold mastery and increase ice bolt instead.

Charged Bolts // Frozen orb works nice too, if you bored of the most typical meteorb etc..