Quote Originally Posted by Sandkat View Post
It is double the mana but only 1 second extra cast time.

Thunder 1:
Cast time - 2.41
Mana - 212
Potency - 30
Dot Potency - 35
Duration - 18
Thundercloud Potency - 240

Thunder 3:
Cast time - 3.37
Mana - 425
Potency - 60
Dot Potency - 35
Duration - 24
Thunder Cloud Potency - 340

its also a 5% chance no matter what Thunder you use.

Also, if you only refresh Thunder during Umbral, you arent wasting any mana.

Blizzard 3 > Thunder 3 > Fire 3 > Fire 1 > Firestarter Proc > 500 mana > Blizzard 3 > Repeat.
Sandkat's post was well written. Thunder 3 adds 100 potency always which scales with int. With thundercloud this happens instantly, but otherwise it still happens as long as the mob doesn't die first.