Due to Hawaii’s fairly constant weather patterns, the Farmers’ Almanac provides an overall climatological summary for Hawaii. This summary should give you a good idea of what type of weather to expect if you are traveling to this great state.
The climate of Hawaii is more than normally pleasant for the tropics. Among its outstanding features are the remarkable variability in rainfall over short distances, the sunniness of leeward lowlands contrasting with persistent cloudiness over nearby mountain crests, equable temperatures, and a general infrequency of severe storms. For most localities in the Hawaiian Islands, the range of normal temperatures between the warmest month (August) and the coldest month (February), is only about 6 to 8 degrees. The daily range of temperatures is also small, less than 15 degrees, while humidity is usually moderate to high. However, due to natural ventilation provided by the prevailing winds, the weather is seldom oppressive.
Hawaii lies within a belt of northeasterly trade winds generated by a semipermanent Pacific high-pressure cell to the north and east. These trade winds prevail throughout the year and profoundly influence the climate. On the Big Island, Hawaii, the entire western coast is sheltered by high mountains. But local mountain circulations affect even places exposed to the trades. Contrary to popular belief, completely cloudless skies are quite rare for most areas.
Over the windward slopes of Hawaii, rainfall occurs as the moist northeast trade winds interact with the mountains, the air is forced to ascend and that produces showers. Mean annual rainfalls, except for the semisheltered Hamakua district, increase from 100 inches or more along the coasts to maximums of over 300 inches at elevations between 2,000 to 3,000 feet, and then decreases to just 15 inches at the summits of Mauna Kea and Mauna Loa. Within the city of Hilo, yearly rainfalls vary from about 130 inches a year near the shore to as much as 200 inches upslope. Trade-wind-induced showers are relatively common.
The leeward areas are sheltered from the trades by the mountains, and are therefore drier. However, thanks to sea breezes, created by the daytime heating of the land, moving onshore and upslope, afternoon and evening cloudiness and showers do occur. The driest locality on Hawaii Island, with an annual rainfall of less than 10 inches, is the coastal strip just leeward of the southern portions of the Kohala Mountains and the saddle between the Kohalas and Mauna Kea.
On Honolulu’s island of Oahu, heavy mountain rainfall feeds the water supply for Honolulu. Oahu is driest west of the Waianae Range, along the coast, where rainfall drops to about 20 inches a year. Daytime showers are usually light, and often occur while the sun continues to shine.
Storms that cross the Pacific Ocean a thousand miles to the north, as well as low pressure or tropical storms, may generate seas that cause heavy swell and high surf. Only a few hurricanes or tropical cyclones have scored a direct hit on the Hawaiian Islands. Most get only near enough for the outlying winds, waves, clouds, and rains to affect the islands, and even then that only happens, on average, once every several years. Except for locally heavy rains, really severe weather seldom occurs.
The contrast between the dry season of May through October and the wet season of November through April, is quite pronounced. Major widespread rains, which account for the bulk of the precipitation for the islands, occur several times during each wet season, but are infrequent in the dry season. Approximately 50 percent of the normal annual rainfall occurs in the three months of December through February, and 80 percent in the six months of the wet season. June and July are generally the driest months for most areas.
During the winter, cold fronts may bring blizzards to the upper slopes of Mauna Loa and Mauna Kea, with snow extending down to 9,000 feet or below, and icing occurring nearer the summit. Intense rains in the October to April wet winter season sometimes cause serious flash flooding. Thunderstorms are infrequent and usually mild, and hail seldom occurs. Both are usually associated with major storms in the wet season. Even more infrequently, a small tornado or a waterspout may do some damage.
HAWAIIAN WEATHER IN A NUTSHELL
You will enjoy Hawaiian weather at its best if you plan your visit during the dry-season months of May through October. The two best months are June and July. The driest and sunniest locations are in the southern and western sections of the islands. Remember these two simple facts and you should enjoy mostly dry, warm weather, with only a slight risk of brief isolated showers. If you plan your Hawaiian visit during the wet-season months of November through April, the overall weather patterns will be more changeable.