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Hawaii - Environment

Hawaii The Hawaiian islands are near the northern margin of the tropics, and because of the prevailing northeast tradewinds and the buffering effect of the surrounding ocean, air temperature at a given location in Hawaii is generally equable. At the Honolulu International Airport, for example, the warmest month of the year is August, which has a mean temperature of 80.5 degrees Fahrenheit, and the coolest month is February, which has a mean temperature of 72.0 degrees Fahrenheit. Air temperature can vary greatly from one location to another in Hawaii. The air temperature in the eight-island group can range from about 95 degrees Fahrenheit at sea level to below freezing at the top of some peaks on the island of Hawaii. In the geologic past, these peaks have been glaciated. Northeasterly tradewinds are present about 85 to 95 percent of the time during the summer months (May through September), and 50 to 80 percent of the time during the winter months (October through April). The tradewinds are occasionally interrupted by large-scale storm systems which pass near the islands. The southwestern parts of some islands receive most of their rainfall from these severe storms, which produce a relatively uniform spatial distribution of precipitation. In general, the northeastern, or windward sides of the islands are wettest.

This pattern is controlled by the orographic lifting of moisture-laden northeasterly tradewinds along the windward slopes of the islands. The winds blow across open ocean before arriving at the islands; when the moisture-laden air mass rises over the mountains, the moisture condenses as precipitation. Maximum rainfall occurs between altitudes of 2,000 and 6,000 feet above sea level, but exact amounts vary depending on the form, location, and topography of each island. Above 6,000 feet, precipitation decreases and the highest altitudes are semiarid. High mountain areas are dry because the upslope flow of moist air is prevented from penetrating above altitudes of about 6,000 to 8,000 feet by a temperature inversion. Areas that are leeward (southwest) of mountain barriers are generally dry because air is desiccated during its ascent over an upwind orographic barrier. This is known as the rain-shadow effect.

The latitude of Honolulu, about 20N, is the same as Calcutta and Mexico City. As a result, there is little change in the length of daylight or the angle of incidence of the sun's rays from one season to another. This factor, plus the state's maritime position, means that there is little seasonal variation in temperature.

It is variations in precipitation that mark the major changes in season on the islands. During the summer, Hawaii is under the persistent influence of northeast trade winds, which approach the islands over cool waters located to the northeast and create characteristic Hawaiian weather--breezy, sunny with some clouds, warm but not hot. In winter, these trade winds disappear, sometimes for weeks, allowing "invasions" of storms from the north and northwest. Honolulu has received as much as 43 centimeters of rain in a single 24-hour period. Hawaiian weather stations have also recorded 28 centimeters in an hour and 100 centimeters in a day, both of which rank near world records.

The topography of the islands creates extreme variations in precipitation from one location to another. Mount Waialeale, on Kauai, receives 1,234 centimeters annually, making it one of the world's wettest spots, and Waimea, also on Kauai, receives about 50 centimeters annually--yet these two sites are only 25 kilometers apart. Within the metropolitan area of Honolulu, it is possible to live near the beach in a semiarid climate with less than 50 centimeters of rainfall annually or inland near Pali on the margins of a rain forest drenched by 300 centimeters of precipitation a year. Unlike the Pacific Northwest, the greatest precipitation on the higher mountains in Hawaii occurs at fairly low elevations, usually between 600 and 1,200 meters.

Much of the volcanic soil is permeable. This allows water to percolate rapidly, draining beyond the reach of many plants. Thus, many areas of moderate to low precipitation are arid in appearance.

The isolation of the Hawaiian islands, coupled with their generally temperate climate and great environmental variation, has created a plant and bird community of vast diversity. There are several thousand plants native there and found naturally nowhere else; 66 uniquely Hawaiian land birds have also been identified. Interestingly, there were no land mammals on the islands until humans arrived.

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Page last modified: 01-11-2017 19:23:58 ZULU