[Ironhawk]

lols - like triple ninja'd

Heat and Cold effect combat by increasing fatigue the further you are away from your "correct" temperature. Units which have cold/heat power also gain that amount for each point of temperature. There are also some unique things like Water Elementals come out as Ice Elementals in cold weather. And rain comes down as snow.

Also, while not directly combat related, some spells like Murdering Winter and Flames from the Sky have a proprotional effect based on the temp of the target province.