considering the low cost, it's damage is reasonable.
There is more to cost than just quantum cost. There is also draw cost. In most decks you can only draw once per turn; if not you have to pay a quantum and card cost in order to do this (precog, sundial, hourglass). Even free cards have cost, the cost of a draw. A card would only be worth using if its effect outweighs both the quantum cost and the draw cost (1 for nearly every card).
Try playing a deck with thunderstorm. How often will you be glad you drew it? Only in rare situations and counterdecks. Usually you will have benefited more if you drew another card; basically, thunderstorm is underpowered.
That's why I want it like this
3
Thunderstorm deals 1 damage to opponent's non flying creatures.
Thunderstorm deals 2 damage to opponent's flying creatures.
Draw a card.
=
No draw cost involved and a deck carrying this will deal 6 damage to non-flying and 12 to flying creatures.
18
is easily affordable since you will pay this cost in small pieces of 3.
This spell would be a great support card for wing shield and it fits well air in speed, direct damage and supremacy over all flying beings