Many weather effects are included to increase the realism of the game. The climate of different parts of the world vary in both average temperature and events such as rain or snow. The heat and cold will effect characters not wearing proper attire, increasing their rate of fatigue and decreasing their other abilities. Visibility will be affected by fog, rain, and other climatic events. Ranged weapons will be affected by the wind, and bowstrings will become damp and unusable in rain unless properly protected. Movement slows in snow and combat is especially treacherous on icy ground. Have fun!
Also, different areas of the world will have different weather at the same time. So a channeller affecting the local weather might only be able to create rain for the surrounding rooms, while clear skies are visible nearby. Further to the north a blizzard may be buffeting a city, while gale force winds blow across the roads connecting the two.