Gay Hawaii Travel Guild

Climate

Kaua'i

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 74 °F (23 °C) between late February and early April, to a maximum of 79° to 80 °F (27 °C) 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 90 °F (32 °C) 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 100 mph (160 km/h) 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 4,000 feet (1,200 m) 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 100 mph (160 km/h) or more.

Road to Hana

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 28 inches (710 mm) 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.[2]

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