(The following discussion of winds in Hawaiian waters is from Weather in Hawaiian Waters by Paul Haraguchi [Honolulu: Pacific Weather, Inc. 1979]. Another discussion of Hawaiian climate can be found in “Climate Controls” by Thomas Schroeder, in Prevailing Trade Winds: Weather and Climate in Hawaiʻi, Sanderson, Marie, ed. [Honolulu: UH Press, 1993]. One of the figures is from David Burch’s Emergency Navigation [Camden Maine: International Marine, 1986]. Another figure is from “Polynesian Voyagers to the New World” by Ben Finney [manuscript].)

The winds in Hawaiian waters, especially in the major channels between islands, are the most important of all the weather elements because of their high frequency of critical speeds and their effect on small craft and marine operations. Many have regretted not considering them in their planning, setting out against the strong gusty winds or being caught in the windy ocean.

A wind rose in an open area in Hawaiian waters not affected by land will show trade winds blowing from the northeast quadrant accounting for about 70% of the total winds. Winds from the other quadrants - southeast, southwest, and northwest - account for the other 10, 10 and 10%, respectively. The average wind speed for winds blowing from the northeast, southeast and southwest, and northwest quadrants are roughly 13, 9, 8 and 8 knots, respectively.

See Figure 2. Wind Rose in Hawaiian Waters

These average wind speeds are deceptively low because they are from a population where low wind speeds are more common than high wind speeds. This pulls the computed average wind speed closer to the low wind speeds. In the averaging process, the extreme values are merged into the average value. The high wind speeds are “lost” in the process, so to speak.

It cannot be emphasized too strongly that the average wind speed should not be used for most marine operations because it isn’t representative of the critical wind speeds in Hawaiian waters. The high speeds are the critical wind speeds for marine activities. The unwary will see his plans suffer and fail.

The following story, although overly simplified, illustrates the danger of using an average value when the average is not the statistic to use. A non-swimmer, 6 feet tall, confidently enters the water with an average depth of only 4 feet. He drowns after stepping into a hole in the bottom deeper than his height. He was unaware that many depths, high and low, went into the computation of the average depth of the water. The aim of this book is to help the reader become aware of the “holes” in the Hawaiian marine weather. This problem of the sometimes not too useful average wind speed is overcome by studying the different wind regimes in detail.

Trade Winds

The most common wind by far in the Hawaiian waters is the trade wind. This almost-persistent wind blowing from the northeast through east direction became known as the trade wind long ago when clipper ships plied the seas with cargo for trading among countries and were dependent on the broad belt of easterly winds encircling the globe in the subtropics for fast passage. In those olden days, as it is today, the trade winds (trades for short) were synonymous with good weather. But, there are exceptional times and places that the trade winds are not bearers of good weather.

The Pacific Anticyclone, a high pressure center of air called “high” for short, is the source of the trades in Hawaiian waters. The seasonal latitudinal positioning of this high is governed by the earth’s annual revolution on its tilted axis (23. 5° from poles) around the sun.

See Figure 3 (Haraguchi)—Earth’s annual revolution around the sun

In summer when the North Pacific Ocean is colder than the North American continent, the high is well developed over the Eastern North Pacific and remains semi-stationary northeast of the Hawaiian Islands. In the opposite season, winter, when the ocean-land temperature relationship is reversed, the mean position of the high is further south and the high is weaker and not as persistent as during summer.

See Figures 4 and 5 (Haraguchi)—Mean Pressure and Wind Flow for July and January.

See Figure 4 (Finney)—An hypothesized canoe sailing route from Hawaiʻi to California, and the migration of the Pacific High.

Figures 4 and 5 show the mean pressure, called isobar, and wind flow, called streamline, charts of the Pacific High in the Eastern and Central North Pacific for January and July, the months representing the opposite seasons. It is an infrequent occasion when the trades in Hawaiian waters cannot be directly related to the outflow from the high. The mean charts show the dominance of the high and outflowing trades in the Eastern and Central North Pacific especially in the summer. The circulation of the high extends over thousands of square miles.

The high can be visualized as a huge dome of heavier sinking air spreading clockwise in the Northern Hemisphere around the high’s center. Because of the strong sinking or subsidence of the air, the area under the center has light winds and low-height clouds with little precipitation outward to within about 300 miles of the center.

The sinking air causes a temperature inversion in a layer called the trade wind temperature inversion. A temperature inversion is a reversal of the temperature in this layer. Without a temperature inversion, the air temperature decreases with increase in altitude. But with a temperature inversion present, the air temperature becomes warmer with altitude in this layer. The number of degrees of temperature increase is an indication of the strength of the temperature inversion. As a general rule, the lower the height of the temperature inversion, the less the precipitation. And conversely, precipitation increases as the temperature inversion becomes higher in altitude. The temperature inversion acts as a lid to cloud top development.

See Fig. 6 (Haraguchi)—Temperature inversion.

This explains, for the most part, the limited precipitation in the area under the center of the high where the air is undergoing greatest subsidence than farther away from the high where there is less subsidence.

The mean trade wind pattern is a smooth version of the actual happenings. At any given time, the wind flow is not as static as it appears in the mean because the high, the source of the trades, is not as static, especially during the winter. In the winter, large and strong high pressure centers move off the Asian continent and travel eastward within a thousand miles north of the Islands to replace or merge with the high northeast of the Islands. The strength and shape of these highs gradually change as they move eastward and trade wind speeds in Hawaiian waters during these times are directly related to the strength, shape, position and movement of these highs passing north of the Islands. Trades are strongest in Hawaiian waters whenever one of these large and strong highs pass north of the Islands during winter.

See Figure 7 (Haraguchi)—A strong high pressure area within 1000 miles north of the Hawaiian islands.

Figure 7 shows an example of the strong high passing north of the Islands causing gale winds in the major channels. Even after the passage of a high north of the Islands, strong trades prevail in Hawaiian waters if the high is strong and large located northeast of Hawaiʻi. Winter months have the strongest trade wind episodes because of the passages north of the Islands by these highs but the average wind speeds do not reflect this—the average wind speed in summer is higher than that of winter in Hawaiian waters.

As the air from the high travels towards Hawaiʻi passing over the warm waters of the Pacific, it picks up and carries with it moisture and microscopic salt particles called condensation nuclei. These condensation nuclei assist in cloud formation as the air ascends the windward mountain slopes in the Islands. Even with moisture picked up from the ocean during its travel to Hawaiʻi, the air over the Islands is not as humid as air over other tropical islands. The relatively drier source of air, the Pacific High, is in part the reason for the lower humidity in trade wind transported air over Hawaiʻi. The sinking of the air away from the high becomes weaker as the air approaches the Islands, and the trade wind cumulus cloud tops become higher enabling more showers to fall from the taller clouds. The depth of the trades varies from 5,000 to 15,000 feet over the Islands.

While the winds are light near the center of the high, winds outside the central area are stronger as they turn outward clock-wise around the high. The speed of the winds around the periphery of the high in most cases is directly related to the central pressure of the high. As a general rule, the higher the central pressure of the high, the stronger the wind speed around the high.

Trade Winds In Hawaiian Waters

In Hawaiian waters, the trades prevail over 90% of the time in June through August and only 40% to 60% of the time in January through March. During summer, the trade wind can persist through an entire month while during winter it sometimes can be absent almost an entire month. The reason for the higher frequency during summer is that the Islands are in the belt of almost persistent trades from the Pacific Anticyclone. But during winter, the mean position of the high is further southeast of the summertime position and the high is not as strong or persistent. Interruptions in the trades over the Islands are much more frequent in winter than summer with intrusions of low pressure systems displacing the trades. These low pressure systems will be discussed later. The table below presents the mean monthly frequency of the trade wind in Hawaiian waters.

Strong gusty trades cause the most problems for mariners in Hawaiian waters. It can be pleasantly brisk and refreshingly cool on land but at the same time blustery in the major channels. The high, especially the migratory high, is the cause of the majority of the gusty trade wind episodes in the Hawaiian waters. The gusty trades occur often—more often than people realize. One out of every four days in a year is affected and it may not be prudent to enter the waters exposed to the strong trades, especially in the major channels, in a small craft during these times. These gusty trades blowing from the northeast through east direction, funnel through the major channels —the Kauaʻi, Kaiwi, Pailolo, Kalohi and ʻAlenuihāhā Channels at speeds 5 to 20 knots faster than the speeds over the open ocean.

Channels of the Hawaiian Islands Exposed to the Trade Wind Flow

Channel	Distance between Island
Kauai (Kaʻieʻiewaho)	63 miles between Kaua'i and O'ahu
Kaiwi	22 miles between Oʻahu and Molokaʻi
Pailolo	7.5 miles between Molokaʻi and Maui
Kalohi	8 miles between Molokaʻi and Lānaʻi
ʻAlenuihaha	26 miles between Maui and Hawaiʻi

Why is it so much windier in the channels? The answer lies in the theory of fluid mechanics. A fluid, in this case, air, accelerates as it passes through a narrow corridor. The channels between the major islands are open to the trade wind direction and act as the corridors for the trades to accelerate through. Also interesting is that coastal waters off northwest Kauaʻi and southeast Hawaiʻi Island experience this funneling effect on the trades although there is only half of the corridor in these cases. [This increase in wind speed is due to the tendency of air flow to accelerate as it “passes over or around island mountains(;) (the wind) accelerates as a result of the pressure of the air overtaking it from upstream.” See Schroeder 22.] The waters just to the lee of the islands are the only protected waters from the strong trades. The largest area of sheltered waters is off Kona, West Hawaiʻi Island.

As the trades speed through the channels, they push on the waters and generate choppy waves which in time become rough and confused. But the waves cannot build up to their maximum attainable height as would happen in the open ocean because of the limited length of channel waters upon which the accelerated trades push. Although the heights of the waves in the channels are not as high as those generated in the open seas by winds of the same speed and duration blowing over a larger area, called the fetch area, the channel waters become hazardous to small craft operation.

A strong trade wind episode in the Hawaiian waters is not a short duration occurrence. This condition persists for several days, in most cases, before tapering off. The reason for this long duration is that the strong outflowing winds from the high which cause the majority of strong trades in Hawaiian waters cover a large area and blow over this area for several days even as the high changes its size, shape, strength and position.

Analysis Of Moderate Trade Winds In Hawaiian Waters

Figures 8 through 13 show the moderate trade wind flow in the Hawaiian waters. The reason for depicting the moderate trade wind condition is that the wind flow pattern will be close to that facing a mariner venturing into the Hawaiian waters on a trade wind day. The main difference between the strong trade wind day and the moderate trade wind day is the strength of the wind speeds. The wind flow pattern remains basically the same.

The wind direction and wind speeds are illustrated around the islands and through the channels in the following figures. The solid lines with arrows depict the wind directions and the dashed lines depict the wind speeds in knots. The trade wind direction prevails except in the lee of the islands. The centers of strong winds are in the major channels and waters off northwest Kauaʻi and southeast Hawaiʻi. In the narrative that follows, unless otherwise noted, the direction of the wind is from the east-northeast direction.

(See Figures 8 through 13 [Haraguchi]—Winds of the Hawaiian Islands.)

Kauaʻi Island and Kaulakahi Channel (Figure 8). The waters off northwest and south Kauaʻi experience wind speeds greater than 16 knots while slightly lesser speeds prevail off East Kauaʻi. Although there is no funneling of the trades between two islands on the north side of Kauaʻi, a maximum wind speed core of greater than 18 knots lies off northwest Kauaʻi.

In the Kaulakahi Channel, between Kauaʻi and Niʻihau, the wind speed minimum is centered off West Kauaʻi because of the sheltering effect of the island on the trades. The trades bend around Kauaʻi from the north and south into the channel with lighter speeds.

Kauaʻi Channel (Figure 9). The trades pass through this wide channel between Kauaʻi and Oʻahu with speeds greater than 16 knots with a large core of speeds greater than 18 knots northwest of Oʻahu.

Oʻahu Island and Kaiwi Channel (Figure 10). In the Kaiwi Channel, the trades accelerate through with speeds greater than 20 knots. The waters off windward Oʻahu receive moderate trades with speeds about 15 knots. Lower speeds occur in partly sheltered Kā+neʻohe Bay. Over leeward Oʻahu waters, there is bending of the trades from the north and south around the island meeting off Waiʻanae-Nanakuli. This area of variable wind direction and light wind speeds at times extend out to 20 miles from shore.

Molokaʻi, Lāna'i, Kahoʻolawe and Maui Islands with Kalohi, Pailolo, ʻAuʻau, Kealaikahiki, ʻAlalākeiki and ʻAlenuihāhā Channels (Figure 11). The waters around Molokaʻi are not sheltered from the trades except partially off Kalaupapa and off Laʻau Point. The trades funnel through the Pailolo and Kalohi Channels with speeds greater than 18 knots with maximum wind speed cores of 20 knots.

Funneling of the trades through the saddle in the Central Valley of Maui between East and West Maui cause higher speeds in the Kealaikahiki Channel, but not as strong as those in the Pailolo and Kalohi Channels. The wind speed off Kahului is faster than over waters off other [areas] of windward northern Maui.

The waters in the ʻAuʻau and ʻAlalākeiki Channels are sheltered from the trade wind by Maui, but there is bending of the trades from the north into ʻAuʻau Channel from the Pailolo Channel and from the south into ʻAlalākeiki Channel from the ʻAlenuihāhā Channel. Lānaʻi shelters the waters off its southwest coast from the trades and there is a sizeable area of minimum wind speed centered off southwest Lānaʻi. Even small Kahoʻolawe shelters a very small area off its west side.

The trades in the ʻAlenuihāhā Channel are the strongest of all the channel winds in the Hawaiian waters. The reasons for the strongest winds are that the dominating tall and massive Mount Haleakala in East Maui and the Kohala Mountains of North Hawaiʻi form a good corridor for the funneling of the trades and the narrowness of the channel. Wind speeds greater than 20 knots extend over the ʻAlenuihāhā Channel with a large maximum core of greater than 22 knots.

West Hawaiʻi Island (Figure 12). Light and variable winds prevail to about 40 miles off southwest Hawaiʻi because of the strong sheltering effect of tall and massive Mauna Kea and Mauna Loa. The Kohala Mountains of North Hawaiʻi somewhat shelter the waters off northwest Hawaiʻi. The trades funneling through the saddle between the Kohala Mountains and Mauna Kea blow offshore through Kawaihae Bay with slightly stronger winds than those over adjacent waters.

East and South Hawaiʻi Island (Figure 13). Wind speeds off East Hawaiʻi are relatively lighter than over the open ocean for about 15 miles seaward from the northern slopes of Mauna Kea to south of Hilo. The minimum wind speed is centered in Hilo Harbor. The probable cause of this lessening of speeds near the east coast is that the trades flowing in perpendicular to the coast, is slowed with the massive mountains, Mauna Kea and Mauna Loa, acting as physical barriers to the perpendicular wind flow at the coast.
The waters immediately offshore of the Kau District off southeast Hawaiʻi is sheltered from part of the strength of the trades while greater speeds lay just offshore. The maximum wind speed core lies parallel to the coast. (This analysis of the mean trade wind flow in Hawaiian waters was made possible by use of National Marine Fisheries data.)