Climatic Regions of the United States, Tropical Rain Forest
Hawaii, a chain of islands more than 3,000 km (2,000 mi) from the mainland, is the only U.S. state outside North America and the only one with a Tropical Rain Forest climate. Hawaii's rain forests thrive in wet windward locations (essentially the northeast sides of the islands). Steady trade winds blowing from the northeast push moist ocean air over the land. As moisture-laden winds rise over the island's volcanic mountains, the air expands, cools, and becomes incapable of retaining moisture. Heavy rains result.
As the same air descends on the leeward side of a mountain (the side away from the wind), it contracts, becomes warmer, and absorbs more moisture; thus rainfall is unlikely. For example, Mount Waialeale, on the island of Kauai, is the wettest place on earth, with an average of 11,455 mm (451 in) of rain annually. Yet the city of Honolulu, located on the dry southern coast of Oahu, faces water shortages with a 640-mm (25-in) average annual rainfall.
Although the islands lie within the hot climate zone of the tropics, the continuous Northeast trade winds produce pleasant sea breezes that moderate the temperature. July temperatures average around 21° C (70° F), and winter temperatures are only slightly lower.
The average soil in agricultural areas is red (due to high iron content), moderately fertile, and relatively productive for agriculture. Most of the soils are highly weathered, and water has leached much of the humus from the soil. The better-watered areas are noted for thick growth of tropical trees and shrubs. In dry areas, the dominant vegetation includes xerophytic plants, which have special characteristics to retain moisture.
The Hawaiian Island chain is one of the most remote groups of islands in the world, isolated from the world's continents by the vast expanses of the Pacific Ocean. This isolation engendered a variety of native plants and animals. Although the islands have a limited variety of species, these exhibit many unusual characteristics. Nearly 15 bird species and all native varieties of plants are unique to the islands. Only one species of mammal, the hoary bat, is native to the island. Because of human carelessness, numerous species of animals and plants are either rare or endangered. Both the hoary bat and monk seal are endangered, as well as 227 species of plants and 250 insects.
Warm temperatures, abundant precipitation, and moderately fertile volcanic soils encourage the growth of tropical fruits and vegetables. Pineapples and sugar cane were the leading crops for a century, but agriculture at the end of the 20th century was more diverse. Specialty crops include papayas, macadamia nuts, and exotic flowers. Because of its tropical climate, Hawaii is the only place in the nation that raises coffee.