The Hawaiian Islands, once known as the Sandwich Islands, form an archipelago of 19 islands and atolls, numerous smaller islets, and undersea seamounts trending northwest by southeast in the North Pacific Ocean between latitudes 19° N and 29° N. The archipelago takes its name from the largest island in the group and extends some 1,500 miles (2,400 km) from the Island of Hawaii in the south to northernmost Kure Atoll. Excluding Midway, which is an unincorporated territory of the United States, the Hawaiian Islands form the U.S. State of Hawaii.
This archipelago represents the exposed peaks of a great undersea mountain range known as the Hawaiian-Emperor seamount chain, formed by volcanic activity over a hotspot in the earth's mantle. At about from the nearest continent, the Hawaiian Island archipelago is the most isolated grouping of islands on Earth.
The Hawaiian Islands comprise a total of 137 islands and atolls, with a total land area of . Except for Midway, which is an unincorporated territory of the United States, these islands and islets are administered as the State of Hawaii — the 50th state of the United States of America.
Some information sources state that there are 137 "islands" in the Hawaiian chain. This number includes all minor islands and islets offshore of the main islands (listed above) and individual islets in each atoll. (Hawaii state government, undated). Following is a list of islets and small offshore islands that make up the total count beyond 19:
Almost all magma created in the hotspot has the composition of basalt, and so the Hawaiian volcanoes are constructed almost entirely of this igneous rock and its coarse-grained equivalents, gabbro and diabase. A few igneous rock types with compositions unlike basalt, such as nephelinite, do occur on these islands but are extremely rare. The majority of eruptions in Hawaii are Hawaiian-type eruptions because basaltic magma is relatively fluid compared with magmas typically involved in more explosive eruptions, such as the andesitic magmas that produce some of the spectacular and dangerous eruptions around the margins of the Pacific basin.
Hawaii (the Big Island) is the largest and youngest island in the chain, built from five different volcanoes. Mauna Loa, comprising over half of the Big Island, is the largest shield volcano on the planet. The measurement from sea level to summit is more than , from sea level to sea floor about .
See also: List of Hawaii rivers
The Hawaiian Islands are the site of many earthquakes. Generally, they are caused by the islands' volcanic foundations. Most of the early earthquake monitoring took place in Hilo, by Sarah J. Lyman and her family. From 1833 to 1896, approximately 4 or 5 earthquakes were reported per year.
The state of Hawaii accounted for 7.3% of the United States' reported earthquakes with a magnitude 3.5 or greater from 1974 to 2003, with a total 1533 earthquakes. Hawaii ranked as the state with the third most earthquakes over this time period.
On Sunday, October 15, 2006, there was an earthquake with a magnitude of 6.7, off the northwest coast of the island of Hawaii, near the Kona area of the big island. The initial earthquake was followed approximately five minutes later by a magnitude 5.7 aftershock. Minor-to-moderate damage was reported on most of the big island, including several major roadways rendered impassable by rock slides, and other structural damage, and effects were felt as far away as Honolulu, Oahu, nearly from the epicenter. Power outages lasted for several hours to whole days on several islands. Several water mains ruptured.
Linda Lingle, the governor of Hawaii made a statewide disaster declaration several hours after the earthquake struck. A tsunami alert was issued, but quickly canceled after sensor buoys failed to detect significant wave activity. No deaths or life-threatening injuries were reported.
The Hawaiian Islands are home to a large number of endemic species. The plant and animal life of the Hawaiian Islands developed in nearly complete isolation over about 70 million years. Mammals were absent until they arrived with the first human settlers.
Human contact, first by Polynesians, introduced new trees, plants and animals. These included voracious species such as rats and pigs, who took a heavy toll on native birds and invertebrates that evolved in the absence of such predators. The growing population also brought deforestation, forest degradation, treeless grasslands, and environmental degradation. As a result, many species which depended on forest habitats and food went extinct. As humans cleared land for farming, monocultural crop production replaced multi-species systems.
The arrival of the Europeans had a significant impact, with the promotion of large-scale single-species export agriculture and livestock grazing. In turn, this led to the increased clearing of forests, and the development of towns, driving more species to extinction. Today, many of the remaining endemic species are considered endangered.
The islands receive most rainfall from the trade winds on their north and east flanks (called the windward side) as a result of orographic precipitation. Coastal areas in general and especially the south and west flanks or leeward sides, tend to be drier. Because of the frequent build-up of Tradewind clouds and potential showers, most tourist areas have been built on the leeward coasts of the islands.
In general, the Hawaiian Islands receive most of their precipitation during the winter months (October to April). Drier conditions generally prevail from May to September, but the warmer temperatures increase the risk of hurricanes (see below).
Temperatures at sea level generally range from high temperatures of 85-90 °F (29-32 °C) during the summer months to low temperatures of 65-70 °F (18-21 °C) during the winter months. Very rarely does the temperature rise above 90 °F (32 °C) or drop below 60 °F (16 °C) at lower elevations. Temperatures are lower at higher altitudes; in fact, the three highest mountains of Mauna Kea, Mauna Loa, and Haleakala sometimes receive snowfall during the winter.
One of the most distinctive features of Hawaii’s climate is the small annual variation in air temperature range. This is because there is only a slight variation in length of night and day from one part of Hawaii to another because all its islands lie within a narrow latitude band. The small variations in the length of the daylight period, together with the smaller annual variations in the altitude of the sun above the horizon, result in relatively small variations in the amount of incoming solar energy from one time of the year to another. This factor, and the location of Hawaii in mid-ocean contribute to Hawaii’s pleasant climate. The surface waters of the open ocean around Hawaii have an average temperature that ranges from 73° to between late February and early April, to a maximum of 79° to in late September or early October. With air temperatures this mild for hundreds of miles around, the air that reaches Hawaii is neither very hot nor very cold. Temperatures of and above are quite uncommon (with the exception of dry, leeward areas). In the leeward areas, temperatures may reach into the low 90’s several days during the year, but temperatures higher than these are unusual.
The other reason for the small variation in air temperature is the nearly constant flow of fresh ocean air across the islands. Just as the temperature of the ocean surface varies comparatively little from season to season, so also does the temperature of air that has moved great distances across the ocean; the air brings with it to the land the mild temperature regime characteristic of the surrounding ocean. In the central North Pacific, the Trade winds represent the outflow of air from the great region of high pressure, the Pacific Anticyclone, typically located well north and east of the Hawaiian Islands. The Pacific High, and with it the trade-wind zone, moves north and south with changing angle of the sun, so that it reaches its northernmost position in the summer. This brings the heart of the trade winds across Hawaii during the period of May through September, when the Trade winds are prevalent 80 to 95 percent of the time. From October through April, the heart of the Trade winds moves south of Hawaii; however, the Trade winds still blow across the islands much of the time. They provide a system of natural year-long ventilation throughout the islands and bring to the land the mild, warm temperatures characteristic of air that has moved great distances across tropical waters.
The wind patterns on the islands are very complex. Though the trade winds are fairly constant in speed and duration, their relatively uniform air flow is distorted and disrupted by mountains, hills, and valleys. The usual regime is to have upslope winds by day and downslope winds by night. Local conditions that produce occasional violent winds are not well understood, even though the general causes of these winds can be surmised. These are very localized winds, observed only in a few areas. They sometimes reach speeds of 60 to and are best known in the settled areas of Kula and Lahaina on Maui. The Kula winds are strong downslope winds that occur on the lower slopes of the west side of Haleakala. These winds tend to be strongest between 2,000 and above mean sea level. The Lahaina winds are also downslope winds, but have somewhat different characteristics. They are also called “lehua winds” after the lehua tree, whose red blossoms fill the air when these strong winds blow. They issue from the canyons at the base of the main mountain mass of western Maui, where the steeper canyon slopes meet the more gentle piedmont slope below. These winds are quite infrequent, occurring every 8 to 12 years. They are extremely violent, with wind speeds of 80 to or more.
Cloud Formation – Under trade wind conditions, there is very often a pronounced moisture discontinuity between 4,000 and 8,000 feet (1,200–2425 m). Below these heights the air is moist; above it is dry. The break (a large-scale feature of the Pacific Anticyclone) is caused by a temperature inversion embedded in the moving trade wind air. The inversion tends to suppress the vertical movement of air and so restricts cloud development to the zone just below the inversion. The inversion is present 50 to 70 percent of the time; its height fluctuates from day to day, but it is usually between 5,000 and 7,000 feet (1,500–2,100 m). On trade wind days when the inversion is well defined, the clouds develop below these heights with only an occasional cloud top breaking through the inversion. These towering clouds form along the mountains where the incoming trade wind air converges as it moves up a valley and is forced up and over the mountains to heights of several thousand feet. On days without an inversion, the sky is almost cloudless (completely cloudless skies are extremely rare). In leeward areas well screened from the trade winds (such as the west coast of Maui), skies are clear 30 to 60 percent of the time. Windward areas tend to be cloudier during he summer, when the trade winds and associated clouds are more prevalent, while leeward areas, which are less affected by cloudy conditions associated with trade wind cloudiness, tend to be cloudier during the winter, when storm fronts pass through more frequently. On Maui, the cloudiest zones are at and just below the summits of the mountains, and at elevations of 2,000 to 4,000 feet (600–1,200 m) on the windward sides of Haleakala. In these locations the sky is cloudy more than 70 percent of the time. The usual clarity of the air in the high mountains is associated with the low moisture content of the air.
It is also true that in Hawaii very light showers are extremely frequent in most localities. On the windward coast, it is common to have as many as 10 brief showers in a single day, not one of which is heavy enough to produce more than one-hundredth of an inch of rain. This is because the usual run of trade wind weather yields many light showers in the lowlands, whereas the torrential rains are associated with a sudden surge in the trade winds or with a major storm. Hana has had as much as of rain in a single 24-hour period.
Major storms occur most frequently between October and March, inclusive. During this period, there may be as many as six or seven major storm events in a year. Such storms bring heavy rains and are sometimes accompanied by strong local winds. The storms may be associated with the passage of a cold front – the leading edge of a mass of relatively cool air that is moving from west to east or from northwest to southeast.
Kona storms are features of the winter season. They are so-called because they often generate winds coming from the “kona” or leeward direction. The rainfall in a well-developed Kona storm is more widespread and more prolonged than in the usual cold-front storm. Kona storm rains are usually most intense in an arc, or band, extending from south to east of the storm and well in advance of its center. Kona rains last from several hours to several days. The rains may continue steadily, but the longer lasting ones are characteristically interrupted by intervals of lighter rain or even partial clearing, as well as by intense showers superimposed on the more moderate regime of continuous, steady rain. An entire winter may pass without a single well-developed Kona storm. More often, however, there are one or two such storms a year; sometimes there are four or five. Three harbors provide some protection from Kona storms Kahului Harbor (used mostly for commercial vessels), Lahaina and Maalea Harbors used primarily for sailing craft.
True hurricanes are very rare in Hawaii, indicated by the fact that only four have affected the islands during a 63-year period. Tropical storms are more frequent. These are similar to hurricanes but with more modest winds, below . Because weak tropical storms resemble some Kona storms in the winds and rains they produce, and because early records do not distinguish clearly between them, it has been difficult to estimate the average frequency of tropical storms. A tropical storm will pass sufficiently close to Hawaii every year or two to affect the weather in some part of the Islands. Unlike cold fronts and Kona storms, hurricanes and tropical storms are not limited to the winter season. They are most likely to occur during the last half of the year, from July through December.
Hawai‘i is protected by the vastness of the Pacific (i.e. the improbability of a direct hit); as storms cross the Pacific they tend to lose strength if they bear northward and encounter cooler water. It is thought that the topography of the highest islands (Haleakalā on Maui, Mauna Kea and Mauna Loa on the Big Island) may protect these islands, and certainly Kauai has been hit more often in the last 50 years than the others.
Despite being a tiny speck of land within the vast Pacific Ocean, the Hawaiian Islands have a surprising effect on ocean currents and circulation patterns over much of the Pacific. In the Northern Hemisphere, the trade winds blow from northeast to southwest, from North and South America toward Asia, between the equator and 30 degrees north latitude. Typically, the trade winds continue on an uninterrupted course across the Pacific — unless something gets in their way, like an island.
Hawaii's high mountain landscape presents a substantial obstacle in the path of the trade winds. The elevated topography blocks the airflow, effectively splitting the trade winds in two. This split causes a zone of weak winds, called a "wind wake," to form on the leeward side of the islands.
Aerodynamic theory indicates that an island wind wake effect should dissipate within a few hundred kilometers and not be felt in the western Pacific. However, the wind wake caused by the Hawaiian Islands extends , which is roughly 10 times longer than any wake observed elsewhere. The long wake testifies to the strong interaction between the atmosphere and ocean, which has strong implications for global climate research. It helps researchers assess climate sensitivity, namely how much increase can be observed in the global mean temperature as carbon dioxide levels increase. It is also important for understanding natural climate variations, like El Niño.
There are number of reasons why this phenomenon has only been observed in Hawaii. First, because the ocean reacts slowly to fast-changing winds, the wind system must be steady to exert force on the ocean, as is the case with the trade winds. Second, the high mountain topography of Hawaii provides a significant disturbance to the winds. Third, the Hawaiian Islands are large in horizontal scale, extending over four degrees in latitude. It is this active interaction between wind, ocean current, and temperature that creates this uniquely long wake west of Hawaii.
In addition, the wind wake drives an eastward "counter current" that brings warm water from the Asian coast to Hawaii. This warm water drives further changes in wind, allowing the island effect to extend far into the western Pacific. The counter current had been observed by oceanographers near the Hawaiian Islands years before the long wake was discovered, but they did not know what caused it.
The Hawaiian Islands can be affected by tsunamis, great waves that strike the shore typically but not exclusively from the north. Tsunamis are movements of the surface layer of the ocean most often caused by earthquakes somewhere in the Pacific. The city of Hilo on the Big Island has historically been most affected by tsunamis, where the inrushing water is accentuated by the shape of the bay on which the town is situated.