0:54
Firestorm is the most powerful spell you can learn in Skyrim, and it causes a massive explosion. Although this is gameplay (and thus the actual lore version could be far more impressive), as an ingame spell, it has a range of 100 feet (or 30.48m).
Using Nuke Map once again and playing around with it until we have an accurate radius, we end up with a fireball yield of 0.0908 kilotons (or 90.8 tons of TNT).
The in-game lorebook Power of the Elements says that this spell has the power to make the sea boil.
So fierce the sea will boil eh? Why not take this at face value (just because)?
I couldn't find the minimum size of a sea, but this is close enough...
Sea
The “seven seas” has been used to describe the world’s great water bodies for a long time. But there are actually about 50 water formations that can be called a “sea,” and they are quite diverse when it comes to their size, location, and ecosystems.
education.nationalgeographic.org The Sea of Marmara will be the example we use, given it's very small. The Sea of Marmara has a volume of 3378 km^3 (or 3.378e+12m^3). The weight of water per m^3 is 1000kg.
M = 3.378e+12 X 1000kg
= 3.37800e15kg
The average temperature of the sea waters of Norway (the closest real life equivilent to Skyrim) is 11°C. The specific heat of salt water at 11°C is 4.003 kilojoules per kg.
E = 3.37800e15 X 4.003 X 11
= 1.48743474e17 kilojoules (1.48743473999999992e+20 joules)
= 35.55054349904397526 gigatons
Final Results
Fire Storm (low end) = 90.8 tons of TNT
Fire Storm (high end) = 35.551 gigatons
