|Hawaii Travel Guide|
Climate of Hawaii
For most of Hawaii, there are only two seasons: summer, between May and
October, and winter, between October and April
Hawaii is in the tropics, where the length of day and temperature are relatively uniform throughout the year.
Uniform day lengths result in small seasonal variations in incoming solar
radiation and, therefore, temperature. On a clear winter day, level ground
in Hawaii receives at least 67% as much solar energy between sunrise and
sunset as it does on a clear summer day. By comparison the percentages
are only 33 and 20 at latitudes 40 and 50 degrees respectively.
The ocean supplies moisture to the air and acts as a giant thermostat, since its own temperature varies little compared with that of large land masses. The seasonal range of sea surface temperatures near Hawaii is only about 6 degrees, from a low of 73 or 74 degrees between late February and March to a high near 80 degrees in late September or early October. The variation from night to day is one or two degrees.
These tracks, the paths taken by eastward migrating high and low pressure areas, generally are between 35 degrees north and 65 degrees north, the latitudes of changeable weather. To the south, and particularly over the subtropical oceans, we often find an atmospheric eddy that rarely changes its position. Sometimes it is called "nearly stationary"and lasts long enough to be called "semipermanent".
The storm tracks and the Pacific High follow the seasonal shift of the
sun, moving north in summer and south in winter. The high tends to be
stronger and more persistent in summer than in winter. Therefore, in winter,
the trade winds may be interrupted for days or weeks by the invasion of
the fronts or migratory cyclones from the northern latitudes and by Kona
storms forming near the islands. Therefore, winter in Hawaii is the season
of more frequent clouds and rainstorms, as well as southerly and westerly
Hawaii's mountains significantly influence every aspect of its weather and climate. The endless variety of peaks, valleys, ridges, and broad slopes, gives Hawaii a climate that is different from the surrounding ocean, as well as a climatic variety within the islands. These climatic differences would not exist if the islands were flat and the same size.
The mountains obstruct, deflect, and accelerate the flow of air. When warm, moist air rises over windward coasts and slopes, clouds and rainfall are much greater than over the open sea. Leeward areas, where the air descends, tend to be sunny and dry. In places sheltered by terrain, local air movements are significantly different from winds in exposed localities. Since temperature decreases with elevation by about 3 degrees per thousand feet, Hawaii's mountains, which extend from sea level to nearly 14,000 feet, contain a climatic range from the tropic to the sub-Arctic.
The climate of Hawaii can be defined by what it has and by what it does not have. It does not have the extremes of cold winters and summer heat waves and it usually does not have hurricanes and hailstorms. However, Hawaii's tallest peaks do get their share of winter blizzards, ice, and snow. Highest temperatures may reach into the 90s. Thunderstorms, lightning, hail, floods, hurricanes, tornadoes, and droughts are not unknown. However, these phenomena are usually less frequent and less severe than their counterparts in continental regions.
The highest temperature ever recorded in Hawaii was 100 at Pahala (elevation
870 feet) on the Big Island of Hawaii on April 27, 1931. The lowest ever
recorded was 12 on Mauna Kea (elevation 13,770 feet), also on the Big
Island, on May 17, 1979.
Over the ocean near Hawaii, rainfall averages between 25 and 30 inches a year. The islands receive as much as 15 times that amount in some places and less than one third of it in others. This is caused mainly by orographic or mountain rains, which form within the moist trade wind air as it moves from the sea over the steep and high terrain of the islands. Over the lower islands, the average rainfall distribution resembles closely the topographic contours. Amounts are greatest over upper slopes and crests and least in the leeward lowlands. On the higher mountains, the belt of maximum rainfall lies between 2,000 to 3,000 feet and amounts decrease rapidly with further elevation. As a result, the highest slopes are relatively dry.
Another source of rainfall is the towering cumulus clouds that build up over the mountains and interiors on sunny calm afternoons. Although such convective showers may be intense, they are usually brief and localized.
Frequently, the heaviest storm rains do not occur in areas with the greatest average rainfall. Relatively dry areas may receive, within a day or a few hours, totals exceeding half of their average annual rainfall.
The leeward and other dry areas obtain their rainfall mainly from a few winter storms. Therefore, their rainfall is usually seasonal and, their summers are dry. In the wetter regions, where rainfall comes from both winter storms and trade wind showers, seasonal differences are much smaller.
The opposite extreme, drought is not unknown in Hawaii, although it rarely affects an entire island at one time. Drought may occur when there are either no winter storms or no trade winds. If there are no winter storms, the normally dry leeward areas are hardest hit. A dry winter, followed by a normally dry summer and another dry winter, can have serious effects. The absence of trade winds affects mostly the windward and upland regions, which receive a smaller proportion of their rain from winter storms.