New Zealand has a temperate, moist ocean climate without marked seasonal variations in temperature or rainfall. The prevailing winds are westerly, with a concentration of strong winds in the Cook Strait area. The generally mountainous nature of the country, however, causes considerable variation in rainfall (e.g., between the eastern and western sides of the Southern Alps), and, by preventing stratification of air into layers of different density, results in an absence of extensive cloud sheets and a consequent high percentage of sunshine. Mean annual temperatures at sea level range from about 15° C (59° F ) in the northern part of the North Island to 12° C (54° F ) in the southern part of the South Island. Mean annual rainfall ranges from around 30 cm (12 in) near Dunedin to more than 800 cm (315 in) in the Southern Alps.