Code:
bool rain = scene.GetRainDensity() > 0.0f;
bool snow = scene.GetSnowDensity() > 0.0f;
bool badWeather = rain | snow;
bool badVision = scene.GetFog() > 0.0f;
Mission.Current.SetBowMissileSpeedModifier(badWeather ? 0.9f : 1.0f);
Mission.Current.SetCrossbowMissileSpeedModifier(badWeather ? 0.9f : 1.0f);
Mission.Current.SetMissileRangeModifier(badVision ? 0.8f : 1.0f);
So, the above lowers arrow and bolt speed by 10% in rain or snow, the range is dropped by 20% in fog for AI. This was in effect but was disabled as it was causing various issues with the AI. There are currently no plans to bring this back or other weather effects. I will, however, bring this up as a community suggestion and let you know about the decision taken.