Annual precipitation around the mountain ranges varies greatly as the local climate is dominated by the eastward movement of depressions and anticyclones from across the Tasman Sea. The Aoraki / Mount Cook massif is a major obstacle to the prevailing westerly winds as they push depressions and associated cold fronts of moist air from the subtropics in the northwest against the mountain range. As the air rises towards the peaks, it expands and cools, and forms clouds. Rain and snowfall are often heaviest around the level and can last for several days if the front is slow-moving.