The eight main islands of Hawaii, which lie just south of the tropic of Cancer, have a tropical climate that is determined by their latitude and oceanic setting and by the influence of the prevailing northeast trade winds.
Average temperatures range between 22° and 26° C (72° and 79° F) throughout the year at low elevations. Lowland temperatures vary only a few degrees from month to month and rarely more than 6° C (10° F) from day to day. Extreme temperatures rarely occur. Daytime temperatures hardly ever rise above 35° C (95° F), and temperatures below freezing are practically unknown at elevations of less than 1,200 m (4,000 ft). Weather conditions above about 2,500 m (about 8,200 ft) can be quite severe, especially during the winter months.
Traditional Hawaiian seasons may be generally classified into two periods. Kau, or the summer period, normally lasts from mid-April until mid-October; ho‘oilo, or the winter season, usually lasts from mid-October to mid-April. Although mild by the standards of temperate areas, the winter season is characterized by slightly lower temperatures than those that occur during the summer, and by frontal or cyclonic storms that can bring strong northerly winds and much rainfall to some areas of the islands.
Trade winds from the northeast sweep across the islands nearly all of the time during summer and about one-half of the time during the winter. However, the mountains tend to block their passage. As they flow up the mountain slopes, the winds precipitate their moisture as rainfall. Descending the other side, subsequently, they are quite dry. On Hawaii and Maui the winds tend to flow around, rather than over, the high mountain masses. Consequently, the higher slopes of Mauna Loa, Mauna Kea, and Haleakala generally have less rain than the lower slopes. In Hawaii the mountain slopes that face northward and eastward receive the full force of the rain-bearing trade winds and are referred to as the windward side. The drier slopes facing southward and westward are referred to as the leeward side. When the trade winds fail, as they often do for short periods during the winter months, the islands may come under the influence of a south or southwest wind, which sometimes brings rain to the leeward side of the islands. These kona winds generally bring uncomfortably warm and muggy weather.
Rainfall varies greatly from place to place and from month to month. In general, the wettest months are November to April. Rainfall is heaviest on the windward side of the islands and lightest on the leeward side. For example, the windward side of Waialeale peak, on Kauai, receives about 11,680 mm (about 460 in) of rain a year and is one of the wettest spots in the world. However, only a relatively short distance to the southwest, on the leeward side of the mountain, annual rainfall averages only 510 mm (20 in). Snow frequently covers the upper flanks of Mauna Loa and Mauna Kea in winter, and some snow may linger on their summits through the summer months. © "United States" © Emmanuel Buchot and Encarta
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