The north island has a different climate, less warm and humid, and is a more pleasant place to go in summer. The chances of rain are less here. In wintertime snow covers much of the island, making hiking impossible. The island is mountainous.
The island is a very good place for hiking, because of its climate and its varied nature and mountains. The Hokkaido Nature Trail, however, is a relatively recent project, started in 2003 and not ready yet.