Hawaiian Islands, 6,500 mi2 (16,800 km2)
Land-surface form.--The Hawaiian Islands occupy a tropical oceanic position just south of the Tropic of Cancer (lat. 23 1/2 N.). The five principal islands and four smaller ones are all volcanoes in various stages of erosion. The islands are hilly and mountainous, especially toward the east. About one-fourth of the area rises less than 650 ft (200 m) from sea level; one-half lies between 650 and 2,000 ft (200 and 600 m) in altitude, and another one-fourth reaches elevations of more than 2,000 ft (600 m). Hawaii, the largest and most easterly of the islands, has peaks higher than 13,000 ft (3,900 m), and some active volcanoes. Coastlines are mostly rocky and rough. Only Oahu and Niihau have much coastal plain. Surface streams are not abundant, because the ground is highly porous, composed as it is of lavas.
Dense rainforest in Hawaii Volcanoes National Park. (Photo: National Park Service.)
Climate.--Hawaii has a tropical climate. The surrounding ocean and highly persistent northeast trade winds maintain almost uniform climate throughout the year. At any given location, temperature and precipitation remain nearly constant year-round, but both vary greatly with altitude and exposure. Average temperatures at sea level range from about 70F (21C) in January to about 75F (24C) in July. Frost is rare below 4,000 ft (1,200 m) and has never been recorded below 2,500 ft (800 m). On the highest peaks, however, snow may fall in any month of the year. Precipitation is heaviest on the windward side of all the islands; lee slopes are semiarid. On Oahu, for example, the trade winds first reach the peaks on the northeast side of the island, and annual rainfall there averages more than 200 in (5,100 mm). Leeward is a rain shadow; annual rainfall at Honolulu averages about 20 in (510 mm).
Vegetation.--Because the Hawaiian Islands are isolated, their flora are unique; many species were endemic before human settlement. Native plants form a variety of community types, including shrubland, forest, and areas of bog and moss-lichen.
Most shrubland lies in coastal lowlands on the lee sides of the mountains, extending to considerable altitudes where rainfall is slight.
Forests grow above the shrubland on the lee sides of mountains, extending to sea level on the windward sides. There are at least four kinds of native forest, reflecting differences in availability of moisture. One occurs on the dry lee sides of mountains up to about 2,500 ft (800 m). Wetter areas up to about 6,000 ft (1,800 m) support a forest that includes one of the principal lumber trees, the ohia; found with it are treelike ferns. A third type of forest grows above the ohia up to 9,500 ft (2,900 m) on Maui and Hawaii. A fourth is characterized by the koa tree, the largest tree on the islands, which reaches a height of 60 ft (18 m) and a diameter of 12 ft (3.7 m).
Shrubs mixed with scattered trees grow on the upper slopes of the high mountains, above the forest zone. Bogs are common in areas of heavy rainfall. Mosses and lichens grow above timberline, where rainfall is low and frost is frequent.
Soils.--The islands have a complex pattern of leached Ultisols and Oxisols, Inceptisols, and rocky highlands and coastlines. Deep Ultisols are widespread on old lavas forming the older islands; young lavas on the younger islands are still rock.
Fauna.--Because of their isolation, the Hawaiian Islands feature a meager but unique selection of fauna. Introduced mammals include the axis deer, Hawaiian wild boar, feral sheep, and goats.
There are large populations of such water birds as terns, tropicbirds, boobies, shearwaters, and petrels. The major islands have such special species as the white-tailed tropicbird, the rare Manx (Newell's) shearwater, and the dark-rumped petrel, all of which seek crater walls for nesting. Native land birds include hawks, owls, crows, warblers, and thrushes. Several species, including the crested honeycreeper and ou, are near extinction. Many species of birds have been introduced.
There are no native snakes and few other reptiles.