The western parts of New Zealand generally experience higher rainfall than the east. This difference is caused by the mountain ranges that form the geological backbone of the country. This is especially true in the South Island, where the prevailing moisture-laden nor’wester wind from Australia blows across the country. As the wind hits the mountain ranges, it rises and cools, losing its moisture in the form of rainfall. Once the wind has passed the high mountains, it becomes hot and dry, bringing little rainfall to eastern areas.