INTRODUCTION Mark Twain describes the Hawaiian Islands as “the loveliest fleet of islands that lies anchored in any ocean.” (4, p. 1) It is where these islands are anchored that makes them especially unique. Thousands of miles from a continent, in the middle of the Pacific Ocean, the Hawaiian Islands are naturally protected from temperature extremes by this huge mass of water (4, p. 1). The highest temperature in recorded history is 100ºF, while the lowest is 12ºF (6, p. 220). In addition, because these island peaks are surrounded by miles of a moisture-rich, flat ocean surface, orographic precipitation is the primary source of rainfall in Hawai‘i (2, p. 59). Where this orographic precipitation falls has a lot to do with the earth’s general circulation. Because the Hawaiian Islands lie in the tropics, they are within the influence of the Hadley Cell—which explains why warm air rises near the equator, moves toward the poles at high latitudes, sinks to the surface near 30ºN latitude and returns to the equator at the surface. As this flow returns from north to south toward the equator, it is deflected by the earth’s rotation and becomes a northeasterly wind (5, p. 13), known as the trade winds. These winds blow through the islands for a majority of the year, and are most common in the summer (92% of the time in August) and least common in the winter (50% of the time in January) (5, p. 14). This persistent northeasterly flow has set up distinct wet (windward) and dry (leeward) areas on almost all of the eight major Hawaiian Islands. However, in addition to wind direction, the amount and intensity of orographic rainfall is also affected by the shape of each individual island and the way their mountains are oriented.  http://www.opl.ucsb.edu/frankandgrace/hawaii/pics/hawaii_map.jpg KAUA‘I The circular island of Kaua‘i is formed by one eroded volcano, whose highest peak reaches 5,243 feet near Mount Wai‘ale‘ale at the center of the island (4, p. 3). Because the island and its only volcano are circular in shape, orographic effects work well for all wind exposures (5, p. 32). In the dominant flow of northeasterly trade winds, the north and east sides of the island, including the county seat of Lihu‘e (average yearly rainfall 43.00”) (3, p. 50), are characterized by a cool, humid environment. As this moisture-laden air is forced to rise up the slopes of Mount Wai‘ale‘ale through orographic lifting, abundant cloud cover and numerous showers to steady rain develop at the summit, as well much of the north and east slopes to within a mile of the shoreline. The south and west slopes of Kaua‘i feature a dry, almost desert-like climate. The Kekaha coastal area, in the shadow of Mount Wai‘ale‘ale in the prevailing northeast trade wind flow, receives less than 20” of rain a year (1, p. 63). Unless moderate to breezy, the trade winds have a hard time making it to this side of the island. On days when trade winds are light, thermal convection sets up, with sea breezes developing in the afternoon. Conditions described in the two preceding paragraphs occur for a majority of the time, with other wind flows occasionally disrupting the mean. Roles are reversed in a south to southwest “Kona” wind direction, with the south and west coast receiving more rainfall than the north and east. Similar changes would occur in an east to southeast or west to northwest flow. But once again, because Kaua‘i only features one circular volcano, any wind direction would effectively create orographic precipitation, explaining why, Mount Wai‘ale‘ale is one of the wettest places on earth. O‘AHU The island of O‘ahu consists of two parallel mountain ranges—Ko‘olau reaching a maximum height of 3,105 feet, while the Wai‘anae rises to 4,020 feet at Mount Ka‘ala (4, p. 3). O‘ahu is shaped like diamond tilted northwest to southeast, and this does not have much effect on orographic effects. However, the orientation of the island’s two parallel mountain ranges creates an interesting distribution of orographic rainfall. Both are aligned northwest to southeast, nearly perpendicular to the prevailing northeast trade winds. There are three rainfall maxima on O‘ahu, two near the crest of the Ko‘olau Mountain Range, and one near the crest of the Wai‘anae Mountain Range (1, p. 62). The two wetter rainfall maxima lie near Ko‘olau because this range is the first to squeeze out orographic effects, as it lies “upwind” of its sister range, Wai‘anae to its southwest. The wetter of the two Ko‘olau maxima is Kahana, along the crest of the range where it is broadest (5, p. 33), while the other lies just leeward of the Ko‘olau crest to the northeast of state capital of Honolulu. Both are the result of strong uplifting of the trade winds along the steep windward Ko‘olau slope (1, p. 62). The last rainfall maximum is near the summit of Mount Ka‘ala in the Wai‘anae Mountain Range, and receives only about a third of the amount of rain as the Kahana maximum. This discrepancy in rainfall maximum goes back to the fact that Ko‘olau lies upwind of Wai‘anae, as the dominant trade wind flow is partially dissected after crossing the Ko‘olau, resulting in less rain when orographically lifed over Wai‘anae (1, p. 63). Differences in windward and leeward rainfall are about the same as Kaua‘i, with windward locales (40”/year) receiving about double the amount as leeward areas (20”/year). Extreme leeward areas (areas leeward of the Wai‘anae Mountain Range), experience near desert-like conditions in the prevailing northeast flow, and depend on infrequent wind shifts to receive rainfall. One winter storm, which disrupts the trade winds, can provide the area half its annual rainfall (5, p. 33). Also, because both mountain ranges effectively block the trade winds from reaching these extreme leeward areas, thermal convection develops almost daily, with sea breezes setting up near the town of Wai‘anae in the afternoon. Because of the orientation of both mountain ranges, a southwest “Kona” flow is the only other very effective orographic wind flow on O‘ahu. MAUI The island of Maui is shaped like a human in fetal position, formed by two, nearly circular volcanoes, the West Maui Mountains (Pu‘u Kukui at 5,788 feet) and Haleakala (Red Hill at 10,023 feet) (4, p. 3). The climate of West Maui is comparable to that of Kaua‘i’s Mount Wai‘ale‘ale, because of its nearly circular shape which allows orographic rainfall at the summit with almost any wind direction (5, p. 34). Although Mount Wai‘ale‘ale holds rank as one of the wettest spots on earth, Pu‘u Kukui, the summit of West Maui, holds the official United States one-month rainfall record because it is easier to reach than the swamps of Mount Wai‘ale‘ale. Unlike Mount Wai‘ale‘ale, there is a massive volcano (Haleakala) to the southeast of West Maui, effectively blocking orographic effects from a southeast wind flow. Haleakala, nearly double the height than any other volcanoes mentioned until this point, features a rainfall maximum on its slope, rather than near its summit. This is because upslope flow here in most wind directions, especially in a northeast trade wind flow, is capped by an inversion near 5,000 feet—resulting in rainfall maxima around 2,000 to 3,000 feet in elevation (1, p. 62). In the dominant northeasterly trade wind flow, the summit, along with the north and east slopes of the West Maui Mountains feature a cool, moist climate as winds flow off a moisture-rich ocean surface, and are then forced to rise orographically. Like Kaua‘i and O‘ahu, the difference in rainfall between windward and leeward locals is usually half. Just like Kaua‘i’s circular Mount Wai‘ale‘ale, it would take a light trade wind scenario in order for thermal convection and sea breezes to develop on the south and west slopes of West Maui. Near Haleakala, rainfall maxima occur on its north and east slopes, while the summit itself and surrounding uplands, features a rainfall minimum because it lies above the trade wind inversion (1, p. 62). The leeward slopes of Haleakala are very dry because of the enormous mass the mountain has to block the northeasterly trade wind flow from reaching this area. Thermal convection and afternoon sea breezes set up daily in a trade wind flow, creating an afternoon rainfall maximum here, while the rest of the island receives most of its rain in the evening through morning. Simply because of its mass, Haleakala is an effective orographic precipitator in any wind direction, but usually does not feature as high of rainfall totals as West Maui or Wai‘ale‘ale as rainfall has a hard time reaching its summit because of its height. BIG ISLAND OF HAWAI‘I The Big Island of Hawai‘i features the most interesting topography of all the Hawaiian Islands, as five shield volcanoes formed this large mass of land that can fit all the smaller islands within it. The two largest and highest volcanoes are Mauna Kea (13,796 feet) and Mauna Loa (13, 679 feet) (4, p. 4). These two volcanoes create a north-south ridge of elevations of 5,000 feet or higher for nearly 50 miles, effectively blocking the prevailing northeast trade wind from reaching the western side of the island. Regardless of shape or orientation of its mountains, the Big Island of Hawai‘i is so large and massive, it is an effective weather maker of it own, orographically and thermally. Like Haleakala, the mountains of Mauna Kea and Mauna Loa feature rainfall maxima on their slopes, rather than at their summits, as they lie above the trade wind inversion. In the dominant northeasterly trade wind flow, a continuous high rainfall belt sets up on the east slopes of the two gigantic mountains (5, p. 35), extending north along the crest of the Kohala Mountains (maximum elevation 5,480 feet), and south to the summit of Kilauea Volcano (elevation 4,091 feet). Heading west, out of this band of rainfall maximum, one will see a dramatic shift from rain forest, to a barren desert-like climate. With no constant supply of moisture, as the large mountains effectively block the trade winds from getting to this side of the island, the leeward or western slopes and shores of the Big Island are some of the driest locations in the state. With its size, the Big Island is also a great land mass for thermal convection to develop. One unique rainfall maximum occurs on the southeast slope of Mauna Loa near Ka‘u. A diurnal rainfall maximum in the afternoon indicates thermal forcing, but orographic lift does not appear to be occurring in the afternoon (1, p. 61), as noted by observing wind direction in the area, which is parallel to elevation contours. Another unique rainfall maximum occurs near Kona, a town always blocked from the trade wind flow by Mauna Loa and Hualalai (summit elevation 8,271 feet). This weather pattern allows for rapid heating of the land and provides conditions for a well-developed sea breeze circulation to form during the day. Because of its sheer size and height of its mountains, the Big Island’s Kona area sea breeze is the most effective thermal rainfall generator in the state, while weak sea breeze circulations on the other islands rarely produce rainfall. This is because of the trade wind air streams being forced around the island, instead of over, and converging on the opposite, or western side of the island, where this sea breezes circulation forms on a regular basis. Simply because of the amount of land and the height of its mountains, the Big Island is an effective orographic rain maker in any wind flow, with orographic precipitation intensities sometimes enhanced because of thermal convections created because of the island’s size. REFERENCES (1) Giambelluca, Thomas & Sanderson, Marie. “The Water Balance and Climate Classification. Prevailing Trade Winds: Weather and Climate in Hawai‘i. Honolulu: University of Hawai‘i Press, 1993. (2) Giambelluca, Thomas W. & Schroeder, Thomas A. “Climate.” Atlas of Hawai‘i, Third Edition. Honolulu: University of Hawai‘i Press, 1998. (3) Nullet, Dennis & Sanderson, Marie. “Radiation and Energy Balances and Air Temperature. Prevailing Trade Winds: Weather and Climate in Hawai‘i. Honolulu: University of Hawai‘i Press, 1993. (4) Sanderson, Marie. “Introduction.” Prevailing Trade Winds: Weather and Climate in Hawai‘i. Honolulu: University of Hawai‘i Press, 1993. (5) Schroeder, Thomas. “Climate Controls.” Prevailing Trade Winds: Weather and Climate in Hawai‘i. Honolulu: University of Hawai‘i Press, 1993. (6) Williams, Jack. The Weather Book: An Easy-to-Understand Guide to the USA’s Weather, Second Edition. New York: Gannett New Media, 1997. |