Highly arid zones (low rainfall in the worldmap) would mean giving a boost to fire to spread (but not start), whereas less arid zones (high rainfall) would penalize fire spreading. The idea is that more rainfall means more ambient moisture (on the ground, air, etc) and small fires are less likely to grow large. I don't like the way tropical rainforests play ATM because the high tree density tends to burn everything with little to no chance of stopping it without forcing the rain.