descr_daytypes.txt looks promising.
Code:
daytype temperate_summer
climates unused1 steppe temperate_deciduous_forest temperate_coniferous_forest unused2 alpine
seasons summer
weatherevents calm 0.25, hazy 0.4, light_fog 0.15, rain_summer_shower 0.13, rain_storm 0.05, hail_shower 0.02
Those bold climates cover a great deal of TATW's map. If I'm reading this right then every summer battle is 20% chance of rain/hail, 55% haze/fog, 25% clear. Weather changes during a battle as well - not sure where that is defined.