Desert Sun / Heart o Winter mechanics and question

[Tuna-Fish]

I dont think it makes heat go up faster in your own dominion, but causes it to go up in provinces close to it too. This is good thing when fightting someone with neutral dominion, as in my experiences the battles tend to always happen in heat. Against cold I have not seen much effect, the cold of enemy dominion probably spreads better than the fire of your neighboring one.