Just how hot is the Second Sun?

[Saxon]

So far, it makes anywhere with my dominion shoot up to 3 heat. Where there are different dominions, it seems to be pushing it up, but it is not maxing out. Against C'tis dominion, it is pretty much up at three everywhere, which I expect is due to their slightly positive heat dominion.

I will have to take a turn and scry Helheim, who I am not currently fighting.

As such, my income has increased somewhat and I think I am hurting the others, but the overall effect and value is tough to judge. The King of Elemental Fire I summoned has much more clearly visible advantages!

Any other information on the exact mechanisim and effects of this spell is highly welcome. Right now, I am not sure it is worth the cost, especially on a small map.