Could be:
- Rain causes some damage to some structures (kilns, house roofs to repair with roofing material, etc).
- Rain fill or dilute the tanning tubes
- Increased number of mushrooms after rain (strange that no one has mentioned this yet)
- Increased chance of being struck by lightning while wearing metal armor in the rain (you're a walking lightning rod)
- Rain-related spells, summoning rain for a short period (or maybe calling the ancestors for rain with numens, if the rain will affect the crops?)
- Increased chance of cave-ins on the first/second levels of the mine during rain due to soil erosion?