Well that battle was in Gol Phalas, which is neutral temp scales. I understand it says it's "most effective" in cold provinces, but that wording implies the inverse as well, as in "weaker in hot provinces", not, "totally ineffectual in temperate provinces".
In fact, it also states "the entire battlefield", rather than specifying "some portions of the battlefield". I think this spell description needs a MAJOR overhaul.
Regarding your capital, I made that post before the next turn brought me next to it - our quickhosting is amazing - though I spend ridiculous amounts of time scripting new spell combos that will fail as terribly as the last spells that had no effect on you.
Oh, and also having a great time here, and learning a lot - if you host again Indy, I'll be sure to join up!