West and south of California, 2,100 miles away, lies Hawaii. Among the 50 states it is the only one surrounded by the ocean. It is the only state within the tropics. Both of these facts contribute significantly to its climate, as do also its division into separate, widely spaced islands and its topographic diversity.
The islands of the state are the eastern-most members of the Hawaiian island Chain. This Chain extends for a distance of 2,000 miles from Kure and Midway Islands at the northwest to the Island of Hawaii at the extreme southeast end. There are six major islands in the state which occupy a narrow zone 430 miles long. From west to east these are Kauai, Oahu, Molokai, Lanai, Maui, and Hawaii. Taken together with the much smaller islands of Niihau and Kahoolawe, their total area is 6,424 square miles, about 30 percent greater than that of Connecticut. The areas and approximate linear dimensions of the major islands are estimated as follows:
| Island Length (Miles) | Width (Miles) | Area (Square Miles) | |
|---|---|---|---|
| Hawaii | 93 | 76 | 4,021 |
| Maui | 48 | 26 | 728 |
| Oahu | 44 | 30 | 602 |
| Kauai | 33 | 25 | 553 |
| Molokai | 38 | 10 | 259 |
| Lanai | 18 | 13 | 141 |
The islands of the State of Hawaii are terrestrial, summit portions of the long range of volcanic mountains that comprise the Hawaiian Chain. Kauai, in the west, is geologically the oldest of the six major islands and is therefore most strongly eroded, as is evidenced by the deeply cut Waimea Canyon in the western half of the island and by the broadly eroded valley lands in the eastern half. Hawaii, in the east, is geologically the youngest. Its dominant physiographic features are the large mountain masses of Mauna Loa and Mauna Kea, both of which rise to over 13,000 feet above mean sea level and both of which have suffered only slight erosion. The four major islands lying between Kauai and Hawaii are intermediate in age and in the amount of erosion to which they have been subjected. All the islands are bordered by coral reefs and all have coasts that consist in part of cliffs, some of which are 300 to 3,000 feet in height.
The mountainous nature of Hawaii is indicated by the fact that 50 percent of the state lies above an elevation of 2,000 feet and 10 percent lies above 7,000 feet. However, the heights of the mountains vary greatly from island to island, as is shown in the following table:
| Island | Maximum Elevation |
|---|---|
| Hawaii | 13,796 |
| Maui | 10,023 |
| Kauai | 5,240 |
| Molokai | 4,970 |
| Oahu | 4,040 |
| Lanai | 3,370 |
Almost half of the area of Hawaii lies within five miles of the coast. Because of this extreme insularity the marine influence upon the climate is very great, yet the mountains, especially the massive ones on Hawaii and Maui, strongly modify the marine effect and result in conditions that are semi-continental in some localities. This results in climatic conditions of great diversity.
It is convenient to recognize seven climatic sub regions in Hawaii. These are defined chiefly by the major physiographic features of the state and by location with reference to windward or leeward exposure. Since one region grades into another, it would be misleading to attempt to draw sharp boundaries between adjacent regions. In general, however, the regions and their characteristics are as follows:
The most prominent feature of the circulation of air across the tropical Pacific is the persistent trade-wind flow in a general east-to-west direction. In the central North Pacific the trade winds blow from the northeast quadrant, and represent the outflow of air from the great region of high pressure, the Pacific Anticyclone, whose typical location is well north and east of the Hawaiian Island Chain. The Pacific High, and with it the trade-wind zone, moves north and south with the sun, so that it reaches its northern-most position in the summer half-year. This brings the heart of the trade winds across Hawaii during the period May through September when the trades are prevalent 80 to 95 percent of the time. From October through April, Hawaii is located to the north of the heart of the trade winds. Nevertheless, the trades still blow across the islands much of the time, though with a frequency that has decreased to 50 to 80 percent in terms of average monthly values.
The dominance of the trades and the influence of terrain give special character to the climate of the islands. Completely cloudless skies are extremely rare, even though much of the time the dense cloud cover is confined to the mountain areas and windward slopes, while the leeward lowlands have only a few scattered clouds. Showers are very common; yet while some of these are very heavy, the vast majority are light and brief — a sudden sprinkle of rain and that is all. Even the heavy showers are of a special character, in that they are seldom accompanied by thunder and lightening. Indeed, a flash of lightening followed by a crash of thunder is infrequent enough so that many people who have lived only in Hawaii have no real notion of the violence of Mainland thunderstorms as evidenced by the lightning and thunder that are their typical accompaniment. Finally, the trade winds provide a system of natural ventilation much of time throughout most of the state and bring to the land, at least in the lower lying regions, the mildly warm temperatures that are characteristic of air that has moved great distances across the tropical seas.
But the trades, though dominant, are not the only major dynamic element in the climatic setting of Hawaii. Major storm systems may influence all parts of the islands, and in some areas there is a local exchange of air between land and sea on a rhythmic, diurnal basis. There is also the fundamental diurnal cycle of night and day, and because Hawaii is in such a low latitude this cycle shows less variation in length than is found in any other state.
Major storms occur most frequently between October and March, inclusive. During this period there may be two, three or even as many as six or seven major storm events in any particular year. Such storms typically bring heavy rains; and are sometimes accompanied by strong winds, at least on a local scale. 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 of from northwest to southeast. The storms may also be associated with a large eddy, or Low, that has been generated in the moving air in somewhat the same manner that an eddy is generated in a flowing river. Moist, warm air swirling into such eddies produces tremendous clouds and torrential rains.
The land-and-sea circulations are on a far smaller scale than the circulations of the major storm systems. The exchange of air often is confined to areas of a few square miles or tens of square miles. Circulations of this kind are most common on south and west coasts, in locations that are to the leeward with reference to the trade winds and topographically sheltered from them. The Kona Coast of Hawaii, the Lahaina District of Maui, the Ewa-Waianae Coast of Oahu and the Barking Sands area of Kauai are among the localities in which these circulations are common. These circulations have a will-marked diurnal rhythm. Between forenoon and early evening air moves inland on a sea breeze. Sometimes these sea breezes are fairly brisk. During the night and until shortly after sunrise the air drifts back from land to sea. The return drift is usually very gentle, so much that it can barely be felt.
Underlying the land-and-sea circulations are the diurnal rhythms of illumination periods which contribute to the march of temperature, the variations in air pressure, variations in cloudiness and rainfall, and the pace and rhythm of many other facets of the changing weather.
There in only a slight variation in length of night and day from one part of Hawaii to another, since the major islands of the state all lie within a narrow latitude band. The variations at Honolulu are therefore generally representative of those of the state as a whole.
The relatively slight variations in the length of the daylight period in Hawaii as contrasted with other states, 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 form one time of the year to another. The solar energy that pours into the upper atmosphere above Hawaii each day varies by a factor of 50 percent from time of maximum, in early June, to time of minimum, in late December. But at Washington, D.C., the daily receipt of energy is three times as great at time of maximum as it is at time of minimum; while at Anchorage it is 20 times as great.
This small variation in solar energy partly explains why seasonal changes in temperature are so slight throughout much of Hawaii. The other principal reason for the slightness of the variation is the virtually 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. And so the air brings with it to the land the mild temperature regime characteristic of the surrounding ocean.
The rugged configuration of the islands produces marked variations in conditions from one locality to another. Air swept inland on the trade winds of as part of storm circulations is shunted one way and another by the mountains and valleys and great open slopes. This complex three-dimensional flow of air results in striking differences from place to place in wind speed, cloudiness, and rainfall. Together with variations in the elevation of the land, it results in differences in air temperature. Thus the climatic pattern reflects not only such dynamic elements as the trade-wind flow, the passage of storms, and the seasonal rhythms of daylight and of solar heating, but also the static element of topography.
The native Hawaiians recognized only two seasons. KAU was the fruitful season, the season when the sun was directly or almost directly overhear, when the weather was warmer, and when the trade winds were most reliable. HOO-ILO was the season when the sun was in the south, when the weather was cooler, and when the trade winds were most often interrupted by other winds. Modern analysis of the climatic records shows the soundness of this Hawaiian system of seasons, although analysis requires a slight modification of the old definitions. Whereas the Hawaiians recognized two six-month seasons, with KAU extending from May through October and HOO-ILO from November through April, it is more accurate to recognize a winter season of seven months (October through April) and a summer season of only five months (May through September). Under this arrangement summer is very definitely the warmer season, the season with an overwhelming dominance of trade winds, and the season when widespread rainstorms are rare. Also throughout the lowlands, summer is the drier season in terms of average monthly rainfall, except on the Kona Coast (leeward coast) of the Island of Hawaii.
In terms of variations in climatic conditions from one part of the state to another, the most striking contrasts are those in rainfall. At one extreme the annual rainfall averages 20 inches and less in leeward coastal areas and near the summits of the very high mountains, Mauna Loa and Mauna Kea. At the other extreme the annual average exceeds 300 inches along the lower windward slopes of these high mountains and of Haleakala and at or near the summit of the lower mountains of Kauai, Oahu, and western Maui. The complexity of the rainfall pattern and the sharpness of rainfall gradients are evident from annual precipitation maps.
The temperature regime is not as variable from place to place as is rainfall; but there are major geographic differences, chiefly as the result of variations in elevations. The upper slopes of the high mountains lie well above the usual cloud zone and there are moderately wide swings in temperature from day to night. Thus in summer at an elevation of 8,000 feet it is not unusual to have daytime temperatures in the middle 60's and nighttime temperatures in the 40's; while in winter the range is typically from the 50's to near freezing or below. Diurnal temperature ranges are far less in the lowlands, with daytime temperatures commonly in the 70's to 80's and nighttime temperatures in the 60's to 70's. Both in the lowlands and at elevations up to about 4,000 feet the temperature differences from winter to summer are only 4° to 8° F in terms of differences in the mean daily maximum and mean daily minimum.
In general, the Hawaiian climatic is characterized by a two-season year, by mild and fairly uniform temperature conditions everywhere except at high elevations, by striking marked geographic differences in rainfall by generally humid conditions and high cloudiness except on the driest coasts and high elevations, and by a general dominance of trade-wind flow especially at elevations below a few thousand feet. How conditions differ in more detail from season to season and place to place is evident from the following summary, which treats specific aspects of the climate, discusses storms and other weather phenomena, and distinguishes among the principal kinds of climatic regions of the state.
An outstanding feature of the climatic regime of Hawaii is the small annual temperature range. In downtown Honolulu the warmest month is August, with an average temperature of about 78°F the coldest, February, around 72°F — the range between the coldest and warmest months averages only 6°F. At Hilo the range is 5.0° F and at Lihue, 8.0°F. It seems likely that the range does not exceed 9°F at any location in Hawaii below and elevation of 5,000 feet. While annual temperature ranges are almost as small as this in a narrow zone bordering the coast of California, throughout virtually all the remainder of the United States they are very much larger.
The extremely equable temperature conditions are associated with the relatively small seasonal variation in the energy received from the sun and especially with the location of Hawaii, in mid-ocean. The surface waters of the open ocean around Hawaii have an average temperature that ranges from a minimum of 73° or 74°F between late February and early April to a maximum of 79° or 80°F in late September or early October. With temperatures almost as mild for hundreds of miles around, even to the north — the air that reaches Hawaii is neither very hot nor cold.
Outside the dry, leeward areas, temperatures of 90°F and above are quite uncommon. In the leeward areas, temperatures in the low 90's may be reached on several days during the year, but temperatures higher than these are unusual.
At times the upper slopes of Mauna Kea and Mauna Loa are covered with snow during the winter. It is possible that under favorable circumstances cold air formed immediately above this snow cover on clear nights drains into local depressions and accumulates to sufficient depths to produce temperatures of 5°F or lower at the higher elevations.
In exposed locations on the mountains, however, the cold air formed locally is often swept away and replaced by the far warmer air that is carried across and around the mountains by the upper winds; and the local winds that tend to move down slope at night and upslope during the day also have the effect of inhibiting the formation of extremely cold air.
At elevations below 1,000 feet, the lowest nighttime temperatures on record have been in the 50's, except in relatively cloudiness areas such as central Maui and the leeward coasts where temperatures in the high 40's have been known to occur. Here again, these are extreme values, and it is possible for several years to pass before temperatures near 50° F are experienced in any locality near sea level.
In general, lowland temperature conditions in all but the driest areas are well represented by the values for Honolulu, Hilo, and Lihue. Conditions in the driest areas are well represented by the values given later for Mana, Kauai; Waianae, Oahu; and Lahaina, Maui — all of which lie on leeward coasts in areas that are very dry and relatively free of clouds.
It is noteworthy that in these dry areas as throughout Hawaii, August and September are the warmest months of the year, August tending to be the warmest on Kauai, the northernmost island of the state, and September the warmest on Hawaii, the southernmost island. This occurrence of highest temperatures in late summer and early autumn is typical of areas that are overlain by fresh ocean air a high percentage of the time.
As in all mountainous areas, the wind patterns in Hawaii are exceedingly complex. Though the trade winds are fairly constant in speed and blow a high percentage of the time across the adjacent water and onto the islands, the relatively uniform trade-wind flow is distorted and disrupted by the mountains, hills, and valleys. In additions, there are local wind regimes may either reinforce or oppose the general flow of air depending on the local circumstances. Finally, in some weather situations the trade winds are replaced by other general winds, some of which are not nearly as uniform in direction or speed. For all these reasons, average wind speed values are informative only in a broad descriptive sense, and it is necessary to consider a variety of wind situations even to begin to describe realistically the true wind conditions.
Over the ocean around Hawaii average wind speeds are highest during the summer trade-wind period. During the summer months (May through September) wind speeds over the ocean exceed 12 miles per hour 50 percent of the time. Eighty to 95 percent of the time these winds are from the northeast quadrant. During the winter (from October through April), when trade winds are not quite as prevalent, wind speeds are in excess of 12 miles per hour about 40 percent of the time. It is during this winter season that light variable winds are most frequent and this more than balances the fact that winter is also the time of occasional very strong winds — winds stronger than those experienced during the summer except with the rare occurrence of a tropical storm or hurricane in the summer period.
When the trade winds are moderate to strong — generally in excess of 14 miles per hour — they dominate the flow of air across wide reaches of the lowlands. In contrast, light trade winds are commonly felt only along the eastern to northern coasts of the islands and in exposed locations in the hills. With such light trades, there are often well defined local breezes along sheltered leeward coasts; and in some areas, as on the Kona Coast of Hawaii and in the Kihei area on Maui, these local sea and land breeze circulations occur not only when the overall wind is light but also when the trade winds are moderate to strong. Indeed, in these locations it appears that the local sea and land breezes may be reinforced by the trades.
Because wind conditions at any one time may vary from place to place, the frequencies at any one point are not representative of conditions everywhere in Hawaii.
On a few exposed headlands and in mountain passes so oriented as to catch and concentrate the full force of the trades, winds above 40 MPH may occur several days each month of the year. In nearly all other locations, however, such winds occur only occasionally, and then only as the result of a major storm, the passage of a cold front, or an unusual local situation.
Major storms are chiefly events of the winter season. They may yield very high winds from any direction. At Honolulu Airport, wind speeds resulting from these storms have on several occasions exceeded 60 MPH over a period of one to two minutes, and 80 MPH momentarily in gusts. It seems reasonable to assume that 100 MPH speeds may have been attained locally near sea lever, although speeds that high have actually been measured only at Mauna Loa Observatory, at an elevation of 11,150 feet. In any major, wind-producing storm that extreme wind speeds may vary radically from one place to another, due to the peculiarities of the storm and to the effects of terrain. It is not unusual to have maximum speeds of only 35 to 40 MPH in one locality and much higher speeds in a restricted area only a few miles away. The nature of these winds and their extent and frequency are discussed in the separate section on storms.
The local situations that produce occasional violent winds are not well understood, even though the general causes of these winds can be surmised. These are local winds, of very limited extent. They have been observed only in a few areas. They must sometimes reach speeds of 60 to 100 MPH, for they have been known to blow down well-rooted trees as well as power lines designed to withstand very high wind loads. It is likely that these winds occur infrequently in many sparsely settled areas on the slopes of the mountains of Hawaii and Maui, of near the mouths of canyons along the base of these mountains. They are, however, known best in the settled areas of Kula and Lahaina on Maui.
The Kula winds are strong down slope winds. They occur in the Kula District along a section of the lower slopes on the west side of Mt. Haleakala. According to observations by inhabitants of the area, the winds tend to be strongest in the zone that lies between 2,000 and 4,000 feet above mean sea level. In this zone there may be episodes of down slope winds with speeds of over 40 MPH as often as twice a year.
However, winds with speeds n excess of 60 MPH probably occur only once every four or five years, on the average.
The Lahaina winds seem also to be down slope winds, but of somewhat different character from those of Kula. In the Lahaina area they have been given the name of "lehua winds" after the lehua tree which grows in that locality and with whose red blossoms the air is filled 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 have been reported from both the western and southern side of the western Maui mountains. They are evidently quite infrequent, occurring every eight to 12 years on the average. When they do occur, however, they are extremely violent, with wind speeds whose effects suggest they may reach 80 to 100 MPH or even more. They have been known to demolish buildings, uproot trees and cause severe lodging throughout whole fields of sugarcane. That they are partly down slope winds is evident from their being hot and dry. The mountains of western Maui are less than 6,000 feet high as contrasted with the 10,000-foot height of Haleakala, and it seems likely that these local Lahaina winds are caused, at least in part, by the funneling of strong trade winds through certain of the mountain gorges.
Aside from the more violent mountain winds, there are light to moderate mountain winds in many areas on the larger mountains of Hawaii and Maui and in a few local areas in the smaller mountains of the other islands of the state. The usual regime is to have upslope winds by day and down slope winds by night, and this circulation takes its place with that of the sea-and-land breezes in coastal locations as a finer, yet important, feature of the overall wind regime of Hawaii.
Because of the diversity of valleys, hills, and mountains, the moisture distribution within the air that moves across Hawaii is far from uniform, even at one level in that air. There are, however, a few major features of that distribution which are evidenced again and again, and which give coherence to the observed variations in humidity and cloudiness conditions.
Under trade-wind conditions, there is very often a pronounced moisture discontinuity at heights of between 4,000 and 8,000 feet. Below these heights the air is moist; above it is dry. The break occurs in association with a temperature inversion that is typically embedded in the moving trade-wind air, and represents a large-scale feature of the Pacific Anticyclone. From the surface up to the inversion, the temperature decreases with increasing height in a quite uniform manner. The moisture is well distributed throughout this lower layer, and the moisture content of the air is relatively high. At the base of the inversion the temperature increases by several degrees, sometimes quite suddenly. These higher temperatures may extend upward for several hundred feet before the temperature begins once more to decrease upward, in the manner that is most usual in the atmosphere. The significance of the inversion climatically is that it tends not only to produce a lower, more moist layer as against a higher, more dry layer, but it also tends to suppress the vertical movement of air and hence to restrict cloud development to the zone beneath the base of the inversion.
When the inversion is present, as it is 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. On trade-wind days when the inversion is well defined the clouds develop most markedly below these heights with only an occasional cloud top breaking through the inversion. These towering clouds form chiefly along the mountains in particular local situations where the incoming trade-wind air converges as it moves up a valley way and is forced up over the mountains to heights of several thousand feet. The overall result is a complicated cloud pattern, typically with scattered to broken clouds above the lowlands at a fairly uniform height on the windward coasts and with more dense cloud masses along the mountains; these clouds are generally of uniform height, but with some smaller masses that bulge upwards.
On days when there is no inversion the vertical development of clouds is much greater. There may then be towering clouds at sea as well as over the land. There is still a tendency for the maximum cloud development to occur along the mountains with individual clouds likely to tower upward 15,000 feet or more. With a storm or other disturbance in or near the islands the cloud pattern may become very complicated, often with very high cirrus clouds in different arrangements, with layered or cellular cloud decks beneath them at heights of 8,000 or 15,000 feet, and with masses of cumulus clouds rising from near the surface to 30,000 to 50,000 feet. At the other extremes, there are occasional days without an inversion when the sky is almost cloudless, and a few times a year it is possible to scan the entire sky without seeing a single cloud.
Under standard definitions of cloudiness, clear represents 3/10 or less of the entire sky dome covered with cloud, partly cloudy, 4/10 to 7/10, and cloudy 8/10 or more. With reference to these definitions, throughout the lowlands in windward areas in the islands the sky is cloudy about 40 to 60 percent of the time during the daylight hours and is clear 15 to 20 percent of the time. In downtown Honolulu, which is to the leeward of a low mountain range, these values are 27 percent cloudy and 25 percent clear during the daylight hours. In leeward locations well screened from the trade winds, as along the west coasts of Maui, Kauai, and Oahu, the percentage of clear daylight conditions ranges from 30 to 60 percent and cloudy conditions decrease to less than 20 percent. However, in these leeward areas as in those to the windward, the cloudiness increases rapidly as the mountains are approached.
In general, windward areas tend to be cloudier during he summer, when trade winds and trade wind clouds are more prevalent, while leeward areas — which are less affected by trade-wind cloudiness — tend to be cloudier during the winter, when general storms and frontal passages are more frequent. Leeward coasts and lowlands are actually sunnier than the foregoing percentages suggest, since a large part of the reported sky cover consists of clouds which lie over the mountains to windward, but may not obscure the sun or the sky directly overhear. The cloudiest zones of all are at and just below the summits of the mountains of Kauai, Oahu, Molokai, Lanai, and western Maui, and at elevations of 2,000 to 4,000 feet on the windward sides of Haleakala, Mauna Loa, and Mauna Kea. In these locations the sky is cloudy more than 70 percent of the time.
In contrast, the areas of least cloudiness include the high mountains above about 8,000 feet. Here the skies are normally clear between late evening and forenoon, but tend to become cloudier during the middle part of the day. The description by Price and Pales of the usual regime at the Mauna Loa Observatory (11,150 feet) is representative of most high mountain locations:
"...a typical day at the Observatory may dawn bright and clear. Visibility is excellent. Peaks on other islands 80 miles distant and more are distinguishable without difficulty. The trade inversion lies several thousand feet below, and trapped beneath it are the clouds and the bulk of the water vapor, dust, and haze. In the clear atmosphere, insolation is intense..."
"By early afternoon, moister air appears to be seeping upward along the mountain. The humidity increases and fractocumuli (broken cumulus clouds) advance up the slopes. In the next hours the observatory may be briefly enveloped in fog or light rain; but by evening the clouds have dissipated and the conditions which opened the day return. Nights are generally clear."
The usual clarity of the air in the high mountains is associated with the low moisture content of the air. Except for occasional periods of a few hours duration, which seem to accompany the arrival of more moist air from below, the relative humidity is generally below 40 percent, and it often falls to 10 or even five percent. Such low humidities are characteristic of the zone above the inversion that separates the lower and upper air.
Below the inversion, the lowlands and along the lower mountains, the relative humidity commonly averages 70 to 80 percent in windward areas and 60 to 70 percent in leeward ones. Winter relative humidities are somewhat higher than summer ones; and in terms of daily variations the maximum values occur with the minimum temperatures, during the late night and very maximum values occur with the minimum temperatures, during the late night and very early morning. Nighttime values in the cooler and rainier areas often exceed 90 percent, but are more frequently between 70 to 80 percent in the leeward lowlands and other drier localities. Afternoon values are commonly between 60 to 70 percent in the windward and uplands regions, not infrequently between 50 to 55 percent in the drier, warmer coastal zones, and seldom fall below 40 percent anywhere at elevations below the trade wind inversion. Thus, except on the high mountains, the general regime in Hawaii is one of high humidities as compared with conditions in most other states.
If the islands of the State of Hawaii did not exist, the average annual rainfall upon the water where the islands actually lie would be about 25 inches. Instead, the actual average is about 70 inches. Thus the islands extract from the air that passes across them about 45 inches of rainfall that otherwise would not fall. That the mountains are dominantly responsible for this added water bonus is evident from annual rainfall maps, which show the tremendous amounts of rainfall deposited in mountainous areas in the average year. In many mountainous areas of the state these depths exceed 240 inches, or 20 feet. At Mt. Waialeale, on Kauai, the annual average reaches the extraordinary total of 486 inches — over 40 feet. This is the highest recorded annual average in the world.
An average of 70 inches of rainfall over Hawaii is equivalent to not quite 8,000 billion gallons of water per year. This is more than 10 times the annual water use of something over 700 billion gallons. According to the 1965 figures, irrigation accounted for 74 percent of this total use; industrial uses (chiefly for cooling) accounted for 19 percent, while the remaining seven percent was used for domestic and miscellaneous purposes.
With actual water requirements running less than 10 percent of the water supplied annually by rainfall, it may seem strange that there are major problems of water supply in many parts of Hawaii. However, not only are there very substantial water losses due to evaporation, transpiration, and runoff, and by percolation into the porous lavas, but the rainfall distribution is exceedingly uneven. There are very few areas elsewhere in the World and none elsewhere in the United States where rainfall gradients are as steep as they are in Hawaii. In a great many places in Hawaii the annual rainfall gradient exceeds 25 inches per mile; that is the rainfall increases 25 inches for each mile transversed along a straight line. In a very extreme instance, the gradient is 118 inches per mile along the 2-1/2 mile line from Hanalei Tunnel to My Waialeale on Kauai. These steep rainfall gradients and the presence of very dry areas as well as very wet ones result in there being abundant water overall, but with great surpluses in some areas and great deficiencies in others. The major problem of water supply is therefore one of distribution rather than one of a general water shortage.
The zones of highest rainfall on the flanks of the large, high mountains of Haleakala, Mauna Loa, and Mauna Kea lie at elevations of 2,000 to 4,000 feet. In contrast, on the remainder of the mountains, all of which are less than 6,000 feet in maximum elevations, the highest rainfall is along or near the crest line. The difference lies in the fact that the incoming ocean air that is the source of rain usually flows across the lower mountains, whereas it largely flows around the higher mountains. It is significant in this regard that the trade-wind inversion, which tends to suppress vertical lifting of the air, is usually above the level of the crests of the low mountains and is never above the level of the crests of the high mountains.
The driest areas are on the upper slopes of the high mountains, on leeward coasts, or in leeward locations in the interior of the islands as in central Maui. In the driest of these areas the average annual rainfall is less than 10 inches; and one area, around Kawaihae Bay near the northern end of the west coast of Hawaii, the rainfall is less than seven inches.
In many areas of intermediate rainfall the natural landscape indicates great aridity. Thus xerophytic (drought-resistant) plants, widely scattered across otherwise barren ground, are sometimes found in areas in which the annual rainfall is 100 inches or more. This seeming anomaly is the result of very high infiltration rates in the volcanic rocks and soils so that the water that falls as rain moves rapidly downward to great depths in the regolith (loose material overlaying solid rock of earth) and is available to the plant only to a sharply limited extent. Areas of this kind are most commonly found on the relatively young island of Hawaii.
Except for the Kona coast of Hawaii, at elevations below 2,000 feet throughout the state winter is the season of highest average rainfall. The contrast is rainfall between the pronounced at low elevations in the areas with low annual rainfall. The contrast is least in areas of higher annual rainfall, as at Hilo. Further, within the extremely rainy areas above 2,000 feet the winter rainfall maximum disappears in many localities and instead there is fairly uniform, superabundant rainfall all months of the year. Thus at Kukui, Maui, at an elevation of 5,788 feet, the average monthly rainfall is 32.81 inches in winter and 33.49 in summer, with April the wettest month (40.78) and October the driest (23.33). The average annual rainfall at Kukui is slightly over 399 inches. Finally, in the very high mountains where conditions are dry, winter is again the rainer of the two seasons, although only slightly so. This is indicated by the observations at Mauna Loa Observatory (11,150 feet), where with an annual precipitation of about 21 inches, winter precipitation averages 1.8 inches per month and summer precipitation averages 1.6.
The Kona coast of Hawaii has a unique seasonal rainfall regime. The summers are wetter than the winters. At Napoopoo, for example, the average monthly rainfall in winter is 2.80 inches, while the average in summer is 3.70. November is the driest month, with an average of 2.06 inches. September is wettest, with an average of 3.90 inches. Napoopoo is at an elevation of 400 feet and has an average annual rainfall of 38.05 inches. The summer maximum on the Kona coast is associated with the facts that in the well-protected Kona coast is associated with the facts that in the well-protected Kona area, which lies to the leeward of Mauna Loa and Mauna Kea, there is a distinctive local circulation. Daytime onshore breezes yield fairly regular and sometimes in summer than winter. The winter storms that contribute the bulk of the average annual rainfall in other lowland regions of the state contribute less rain to the Kona coast.
In the lowlands at all times of the year, rainfall is most likely to occur during the nighttime or in the morning hours and least likely to occur during midafternoon. Correspondingly, not only is rainfall more frequent at night, but also the total nighttime fall averages more than the total daytime fall when the two 12-hour periods are compared. The values for Honolulu are representative. They show that 59 percent of the annual rainfall total falls, on the average, between 8 p.m. and 8 a.m. In general, this diurnal variation is far more pronounced in summer than in winter, especially with respect to rainfall frequency. Thus in summer, rainfall during the early morning hours (3 to 8 a.m.) is about twice as frequent as during the late morning and early or middle afternoon (11 a.m. to 4 p.m.)
The more pronounced diurnal variations in summer is associated with the fact that most summer rainfall consists of trade-winds showers and these showers are most apt to occur at night. On winter most of the rainfall is the lowlands occurs in general storm situations, and these are as likely to take place during the daytime as at night.
It seems reasonable to suppose that at elevations of a few thousand feet, where trade-wind showers contribute very substantially to the total rainfall in winter as well as summer, the nighttime rainfall maximum is better marked in winter than it is in the lowlands. However, on the very high mountains above the trade-wind inversions, the diurnal cycle appears to be reversed since, judging from the Mauna Loa Observatory observations, the late afternoons are the time when light rains are most apt to occur as accompaniment to the arrival of moist air carried upward by the upslope wind from below.
In most parts of the tropics rainfall is highly variable from one year to another and Hawaii is no exception. Even in areas where the rainfall is very high and the monthly averages are above 10 inches, the rainfall of particular months may vary by 200 to 300 percent from one year to another and there may be occasional months with only one or two inches of rain.
The great variability in annual rainfall is evident from the extreme values at several different stations during their periods of record. In downtown Hilo, in 79 years of record, the highest annual total was 207 inches; the lowest 72. At Honolulu, in 62 years, the extreme values were 46 inches and 10 inches; at Mana, Kauai, in 61 years, 48 and five inches. During a period of only 23 years at Kukui, Maui, the extremes were 578 and 250 inches.
Rainfall variability is far greater during the winter, when occasional storms contribute appreciably to rainfall totals, than during summer, when trade-wind showers provide most of the rain. In January, Hilo has received less than 0.2 inches of rain, but also more than 50 inches. Honolulu has received as little as 0.12 and as much as 18 inches. The highest January total in 23 years at Kukui was 58 inches. The extreme August rainfall totals have a much smaller range. At Hilo the range was from two to 38 inches. At Honolulu it was from 0.2 inches to four inches. At Kukui during 23 years it was from 10 to 88 inches.
With such wide swings in rainfall it is inevitable that there are occasional doubts, sometimes with severe economic losses. The real drought years are the ones where the winter rains fail, in which there are only one or two or even no rainstorms whatsoever on real magnitude. Although such a deficit of winter storms can affect any portion of the state, it hits hardest in the normally dry areas which depends chiefly on winter rains and receive little rain from the trade wind showers. In these localities the small amount of rainfall that occurs during the usual dry summer season is insufficient to prevent severe drought. Severe droughts are usually limited to an area of 50 square miles or so, but even so they may be very costly because of the increased water costs. Where irrigations is not practiced they may destroy r severely damage the crops and range grasses.
Among the winters of record when the rainfall was decidedly below average throughout most of Hawaii was that of 1925-26. At Honolulu during this period every month from November through April experienced below average rainfall and the total rainfall for the five-month period from January through May was less than three inches. For 100 consecutive days during the drought there was not as much as 0.3 inch of rainfall on any one day at Honolulu and only on two days was there as much as 0.2 inch. Daily rainfall amounts of this magnitude are insignificantly small so far as any real benefit to crops are concerned, especially in a tropical area where water losses through evaporation and transpiration are high. During this drought period there were severe water shortages in many areas on Hawaii, even though some areas received appreciably more rain than Honolulu. Drought damage was greatest on rangeland and in truck crop and pineapple areas, where irrigation was not being practiced.
Torrential rainfall is common in all parts of Hawaii except the very high mountains, although it has been known to occur even there. Yet it is also true that in Hawaii very light showers are extremely frequent in most localities. On windward coasts, for example, it is usual to have six, eight, or even 10 brief showers in a single day, not one of which is heavy enough to produce 0.01 inch of rain. This seeming contradiction is explained by the fact that the usual run of trade-wind weather yields many light showers in the lowlands, whereas the torrential rains are associated with other weather regimes: with a sudden surge in the trade-winds or with a major storm.
Extreme rainfall intensities are high in Hawaii. To take the most extreme instance of record, during the storm of January 24025, 1956, over 38 inches of rain fell at the Plantation Office, Kilauea Sugar Plantation, Kauai, within a 24-hour period, out of a storm total of 43.5 inches. During the same storm six inches of rain fell during a single 30-minute period and about 12 inches fell in one hour. The 38-inch value for 24- hours is conservatively low, because the gage was already overflowing when it was emptied for the first time. The six-inch value is correct within one or two tenths of an inch; the 12-inch value for one hour is an estimate only — again because of overflow — and may be in error by as much as an inch.
Rainfall intensities and totals as high as the extraordinary values just cited appear to have occurred when a current or moist, unstable air, converges as it moves up a narrowing valley, and at the same time is forced to rise abruptly over steep mountains. Such special topographic circumstances are not essential to the occurrences of torrential rains. In November 1931 Moanalua, Ohau received 15.2 inches of rain within three hours; Hana, Maui has had as much as 28 inches of rain in 24 hours; Opaeula, Oahu, 26 inches; and Hilo 19 inches, even though none of these are favorably situated with references to topographic surroundings conducive to the occurrence of very high rainfall intensities. In fact, the most copious rains in Hawaii frequently do not occur in localities having the greatest average rainfall; nor is it uncommon during such storms for relatively dry areas to receive within a single day, or even a few hours, totals approaching their mean annual rainfall.
At Honolulu the greatest 24-hour fall in 55 years of record was that of March 5-6, 1958, when 17.41 inches of rain was recorded at the Federal Building in the downtown area. The gage where this total was recorded is situated well away from local topographic influences that might cause excessive rainfall, a fact that was evident from the distribution of maximum storm amounts. The second greatest fall of record in downtown Honolulu is 13.52 inches in 24-hours; the third, 8.07 inches. In general, all stations in Hawaii which have 50 years of record have experienced daily rainfalls of at least eight inches, and the majority have experienced falls of 12 inches or more. These values are comparable to or greater than those for the great majority of stations in the Gulf of Mexico area, which is the principal area in the Mainland United States in which extreme rainfalls are of the same magnitude as those in Hawaii, especially for periods of a few hours or more.
Torrential rains, falling on Hawaii's steep slopes and small drainage basin, often generate flash floods that erode fields and cause landslides and damage to homes. The last of these is an increasingly important problem, as residential development encroaches on flood plains.
Mountain slopes and crests within the cloud belt are frequently exposed to contact with fog or cloud mists. Although no measurable precipitation may be recorded by rain gages, experiments atop Lanai Hale, highest point on the island of lanai, suggest that this "fog drip" may contribute two-thirds as much water to vegetation and soil in that area as does rainfall itself — and proportionately more when rainfall is light.
For many purposes it is convenient to think of Hawaiian weather in a very much simplified way, in terms of trade winds and non-trade-wind conditions. Most of the time the weather is dominated by the trade winds. There are light to moderate trade-wind showers on windward coasts. In the rainier mountain areas these showers are often moderate to heavy and may merge to produce continuous rain for hours or days. In the leeward areas occasional light trade-wind showers may drift over from the mountains to windward, except that in such completely sheltered lees as the Kona Coast of Hawaii, sea breezes ascending the mountain slopes may produce light to occasionally heavy showers in the late afternoon and early evening. On the upper slopes of the highest mountains conditions are dominantly clear except for occasional intrusions of moist air from below, when light rains may fall, especially in the afternoon.
Non-trade-wind conditions are quite different. When the winds are light and variable, moist air may move slowly onto the islands, typically from the southeast or south or southwest. Then there may be intense local storms, sometimes accompanies by lightning and thunder. At other times there may be major storm disturbances, as when a cold front crosses the islands or when a large storm system passes by. Major storms may produce heavy rains in many localities, and even on several different islands, all at the same time. They produce snow and ice in the high mountains, and sometimes even hail. Storm episodes, either on a minor or a major scale, usually occur during a breakdown in the trades (although some of Hawaii's heaviest rains have occurred when trade winds were overlain by low pressure areas higher in the atmosphere); and though they are relatively infrequent, they are an important and integral part of the total weather scene.
Intense local rainstorms other than those that occur under trade-wind conditions are small features that seldom cover more than a few square miles and some times less than a single square mile. They occur most typically in the late afternoon or early evening. In some areas in which there are well-developed sea breezes, such as Kona, Hawaii, they are common occurrences, especially in summer. In most areas, however, they are apt to occur on only a few days per year when the over all winds are light and variable or when there is a gentle flow of air from a southerly direction. Storms of this character may occur at any time of the year.
Intense local storms are sometimes accompanied by lightening and thunder. These thunderstorms also occasionally accompany very intense rainfall along a cold front moving across the islands. Lightening and thunder of the violence common in most areas in the conterminous United States are, however, quite rare. In the lowlands storms accompanied by audible thunder occur only five to ten times a year, on the average. Thunderstorms are reported somewhere in the state about 20 to 30 days a year, and more often in winter than in summer. Judging from the fact that lightning is visible over the mountains decidedly more often than over more frequent than in the lowlands. Even in the mountains, however, tremendous claps of thunder are rare.
Waterspouts and other funnel clouds are not uncommon in the Hawaiian area, about 20 of them being sighted in the average year. Often they are accompanied by towering cumulus clouds and rain, although they have also been observed under trade win conditions. Occasionally a waterspout will drift onshore and do some minor damage, and small tornadoes have been known to form over the islands with the same effect; but these appear to be far less violent than their counterparts in the conterminous United States.
Hail falls somewhere in Hawaii between five and ten times in the average year. Almost always it is quite small — one-fourth inch or less in diameter — but on several occasions hail the size of marbles, and discs about five-eights of an inch in diameter, have been reported. Trees and leafy crops have been battered by hail from time to time, but this is an infrequent occurrence and the area affected are small. Many lifelong residents of Hawaii have never experience hail in these islands.
There are four classes of disturbances that produce major storms. Sometimes a cold front sweeps across the islands bringing with it locally heavy showers and gusty winds. Sometimes a storm eddy, or low pressure system, moves past bringing widespread heavy rains, often accompanied by strong winds. These Lows are known as Kona storms, a term originally reserved for the slowly moving subtropical cyclones that occasionally enter the Hawaiian area, but now increasingly applied by the local public to any widespread rainstorm accompanied by winds from a direction other than that of the trade winds. They include situations in which the low-pressure area is not well marked, but instead consists of a low-pressure trough into which air feeds from either side. The third class of disturbance is the true tropical storm or hurricane. These are rare, but may pass close enough to the islands to yield heavy rains, high winds, and great waves upon the coasts. Also meriting treatment as a separate and fourth class of disturbance are those instances of severe weather attributable to low pressure systems (Lows and troughs) in the upper atmosphere that are not associated with the foregoing cold fronts, Kona storms and tropical storms or hurricanes. Of particular interest are those which occur when the presence of trade winds in the lower atmosphere would appear to contraindicate the likelihood of extensive heavy rain.
These four kinds of major disturbances are not always well defined. Nonetheless they are convenient classes for descriptive purposes.
During some winter seasons as many as six or eight well-defined cold fronts may sweep across two or more islands of the state. During others, there may be only one or two. Because of its location at the northwest end of Hawaii, the Island of Kauai has a few more cold front storms each year than do Oahu and the other islands. Sometimes the southernmost edge of a mass of cool air will barely encroach upon Kauai and will move eastward without reaching as far as Oahu. At other times, cold fronts that pass Oahu may dissipate before reaching the southern islands.
When a cold front passes there may be heavy rains. Typically these rains are spotty, with several inches falling in some areas and only fractions of an inch in others. Winds are gusty, with wind directions mainly from the north and northwest. The cool air that follows the passage of the front brings relatively cloudiness skies. In the lowlands, nighttime temperatures may drop to 62° or 64° F., and in extreme instances to 60° F or slightly lower. Afternoon temperatures are commonly in the middle 70's.
The winds accompanying a cold front are usually brisk, but they may be more than merely brisk in some localities. Perhaps once every three or four years on the average a cold front storm will produce winds that blow down trees or rip the roofs from houses within a limited area of a square mile or so. From time to time cold front passages have given rise in the vicinity of the islands to waterspouts or to a small tornado that move as far southward as the high mountains of Haleakala, Mauna Kea and Mauna Loa, sometimes deposit snow on their upper slopes.
Kona storms, like cold front storms, are features of the winter season. They are so-called because they often bring winds from "kona" or leeward directions. The rainfall in a well-developed Kona storm is more widespread and more prolonged than in the usual cold-front storm; more prolonged, and not as strong as the more extreme winds of the cold front. 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. An entire winter may pass without a single well-developed Kona storm. More often, however, there are one or two such storms a year, and sometimes there are four or five.
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 upon the more moderate regime of continuous, steady rain. The storms of November 1955 and of March 1958 are examples of two different types of Kona storms that brought extreme amounts of rain.
On November 8 and 9, 1955, a major storm system was centered about 275 miles north-northwest of Kauai, Warm, moist air, moving around this storm system, poured across Kauai. Heavy rains began on Kauai about noon on the 8th. These rains continued for 20 to 24 hours and produced rainfall that totaled three to 10 inches throughout the eastern and northeastern sections of the islands. This particular band of very heavy rains moved southeastward down the island chain, with totals of three or four to over 12 inches of the islands of Oahu, Molodai, Lanai, Maui, and Hawaii on November 9 and 10. Meanwhile, the storm system had moved in such a manner as to bring a new flow of fresh, moist air across Kauai, On November 11 and 12 there were tremendous rains on Kauai, bringing total falls of up to 20 inches in 14 hours. This second rain episode, coming shortly after the heavy rains of two days before, caused severe local flooding. Together the two rainstorms caused damage of more than $100,000 on Kauai alone.
The band of torrential rainfall that produced the second storm episode on Kauai also moved down the island chain, but with distinctly less rainfall than had occurred before. Totals varied widely, from no rain at all in some localities to as much as five or six inches in others.
The storm of March 1958 was different in character. On the evening of the 4th, moist air from the south began to flow across Oahu with the air edging inward along a line of convergence. This was a slow, steady flow, without high-speed winds, but as the air continued to converge clouds built up to heights of 40,000 to 50,000 feet and torrential rains set in. Aircraft pilots reported that the clouds lay in a concentrated band squarely across the island. At first the rains were moderate, through steady. The very heavy rains began about 1 o'clock on the morning of the 5th and continued with only brief periods of slackening off until 6 o'clock on the morning of the 6th. During the storm the rainfall totals on Oahu ranged from five to 24 inches, with 17.41 inches in a 24-hour period in downtown Honolulu. There are severe local flooding in several different areas. Mountain and hill slopes still in their natural vegetated state stood up amazingly well. Damage from this storm totaled $400,000.
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 74 MPH. 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. However, studies based on weather satellite photographs now suggest that, on the average, 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 front 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.
Severe weather in the Hawaiian area is at times attributable to low pressure areas which are well developed in the upper atmosphere (rather than near sea level) and are not related to the cold fronts, Kona storms or tropical storms mentioned not related to the cold fronts, Kona storms or tropical storms mentioned previously. The weather which accompanies these upper Lows or trough — towering cumulus clouds, thunderstorms, intense and widespread rain — often resembles that of a Kona storm and may be mistaken for one, except for the absence of the persistence, and sometimes strong, southerly winds that frequently accompany Kona storms. The surface winds during these events may be from almost any direction, and are often associated with low pressure aloft.
Several times a year, on the average, and almost always between October and May, major storms may deposit a foot or more of snow on the upper slopes of one or more of Hawaii's highest mountains: Haleakala, Mauna Kea and Mauna Loa. After an especially heavy fall patches of snow may extend to as low as 7,000 feet, but snow below about 11,000 feet quickly vanishes, since even winter air temperatures in the Hawaiian area are ordinarily above freezing even to altitudes of 13,000 feet and higher. Hence there is no continuous snow cover on these mountain summits. Once every few years, a Kona storm at elevations above 7,000 or 8,000 feet, coating roads and structures thickly with ice and snapping power lines. However, the intense solar heating and the warm air even at those elevations, eliminate all traces of even the most sever ice storms within a few days.
In terms of dollar income the Federal Government is the principal employer, chiefly because of the importance of Hawaii as the Pacific center of the military departments. Aside from these activities, the economy of the islands is based chiefly on agriculture and secondarily on commerce and industry, including tourism. The locations of Hawaii in the mid-Pacific is the primary reason for its importance as a logistic and strategic site for the development of military establishments. However, the mild weather and excellent flying conditions contribute to this importance since they provide preferred sites for many kinds of installations.
The principal agricultural commodity is sugar cane. Cane occupies about 235,000 of the 1.3 million acres of agricultural land. It is raised on all the islands except Lanai and Molokai. It accounts for about 50 percent of the dollar value of all crops. Sugar cane is a two-year crop in Hawaii and most of it is raised under irrigation. The abundance of water in the state and the genial warmth form the climatic basis for this crop. Pineapple is the second most important crop. It occupies about 70,000 acres and is raised on all islands except Hawaii. It occupies lands that are generally dry since the crop requires little water and since low cloudiness and high sunshine are essential during the ripening period. The crop requires 18 to 20 months between planting and harvesting.
Sugar and pineapple lands are in the lowlands, generally below 2,000 feet. In contrast, the rangelands that support the livestock industry chiefly occupy the higher elevations. About one million acres are in ranchland, mostly on Hawaii and Maui. Except where small herds are maintained for dairying, these are the drier lands. Other agricultural activities of the state include dairying, poultry raising, and the raising of tropical fruits, nuts, coffee, and vegetables. Coffee is raised on the Kona coast, where the wet summer and the dry period of early winter are especially favorable to its production. Vegetables include almost the full range of truck crops that are common on the Mainland, although the quantities raised are not sufficient to meet the needs of the local economy.
The climate is a principal basis for the tourist industry of the state. The mild temperatures are an asset as are the low rainfall and generally sunny conditions in such leeward locations as Waikiki and Kona. The trade-wind breeze is also an important element because on the warmer days it often lowers the sensible temperatures very appreciably, making conditions for more pleasant than the temperature and humidity values alone would indicate. In the Honolulu area, for example, temperatures of 80 °F or above occur less than one-half of one percent of the time under conditions when the humidity is as high as 70 percent and there are not at least moderate trade winds. August, September, October and November are the only months when such conditions occur more than once percent of the time.
In general, the climatic conditions in Hawaii, especially in the lowlands, favor many kinds of agricultural, commercial, and industrial enterprises. The summers are warm but not hot, so that air conditioning is a luxury rather than a necessity — although most of the new office buildings, hotels and apartment houses now have central air conditioning. The winters are warm and central heating is unknown except in the mountains. There is abundant water for domestic, industrial and agricultural uses in all but the very driest parts of the state.