A spine of east-west mountains, the New Guinea Highlands, dominates the geography of New Guinea, stretching over from the 'head' to the 'tail' of the island, with many high mountains over . The western-half of the island of New Guinea contains the highest mountains in Oceania, rising up to high, which is even higher than Mont Blanc in Europe, ensuring a steady supply of rain from the equatorial atmosphere. The tree line is around elevation and the tallest peaks contain permanent equatorial glaciers—which have been retreating since at least 1936. Various other smaller mountain ranges occur both north and west of the central ranges. Except in high elevations, most areas possess a warm humid climate throughout the year, with some seasonal variation associated with the northeast monsoon season.