2:52
The Weak Fireball spell causes a small explosion, which in terms of gameplay is 5 feet in radius (as you can see above compared with a troll). 5 feet is 1.52400m. Enter this into Nukemap and we have a yield of 0.0000506 kilotons for a radius of 1.52m, or 0.0506 tons of TNT.
Final Results
Weak Fireball = 0.051 tons of TNT
What's more, replaying Oblivion and if you case a Weak Fireball at a group of loose boulders you'll send them flying with great force, so it's likely even higher.
