The climate of the Hawaiian Islands chiefly depends upon their insular location just within the Tropic of Cancer over 2000 miles from the mainland in the zone of the northeast trades, the altitude, and the effects of cyclonic fronts that pass generally to the northward.
The temperatures are pleasantly mild, averaging about 72° F for all available stations, because the steady blowing trade winds have crossed a moderately cool ocean for thousands of miles ( Fig. 100 ). Differences between the warmest months, August and September, and the coldest months, January and February, are 5 to 8 degrees, with the larger range occurring on the leeward side of mountainous islands. The diurnal range of temperature is about 10° F and exceeds the annual range, which for all stations averaged together is 68.7° F in February, the coldest month, and 75.8° F in August, the warmest month. Altitude has marked effects on temperature, which drops about 3° F for each increase in elevation of 1000 feet. Thus Hilo, near sea level, has a mean annual temperature of 72.1° F, and the Volcano House, elevation 4000 feet, 60.6° F. Frost rarely occurs below 4000 feet elevation above sea level, and has never been known lower than 2500 feet. The summits of Mauna Kea and Mauna Loa on Hawaii are covered with snow frequently in winter; snow sometimes falls on Haleakala on Maui. Uncomfortably hot, humid weather is rare and results from the passage north of Hawaii of frontal disturbances that cause replacement of the trades by Kona (southerly) winds of high humidity, thereby bringing heavy rainfall to the ordinarily dry, leeward side of the islands.
The rainfall is closely related to the mountains and exposure to the winds from the ocean. On windward slopes rainfall increases from sea level to elevations of 3000 to 6000 feet, after which it declines progressively at loftier heights. On Hawaii the zone of maximum rainfall usually lies between about 2500 to 3000 feet elevation on the windward slope of Manua Kea, where it averages 100 to over 300 inches annually, but on Kauai and the dome of west Maui the rainfall continues to increase upwards almost to the summit. Mount Waialeale, Kauai, at an elevation of 5075 feet, has an annual rainfall of about 460 inches, with 624 inches falling in the rainiest year on record, making it one of the wettest spots on earth. Few places in the world equal the Hawaiian Islands in the great contrasts in annual rainfall between the windward and the leeward slopes of a mountain range or dome. On Kauai the summit rainfall of over 450 inches declines to 20 inches annually 15 miles southwest. The windward side of Haleakala up to elevations of 5000 feet has rainfall of 200 to 300 inches annually, declining towards the summit and lessening down the leeward slopes until, near sea level, the rainfall is less than 20 inches per year. Low islands, like Niihau and Kahoolawe, the low isthmus of central Maui, and the low ends of other islands resemble the leeward side of all the islands in having little rainfall. For example, Lahaina, Maui, has 13 inches of rain annually, and Puako, Hawaii, in 1947 recorded only 4.1 inches. Some of the rainfall is connected with cyclones, and at Midway, which is in a latitude several hundred miles north of Honolulu, most of the rains in winter come as a result of such frontal disturbances. Very heavy local rains sometimes occur during the passage of fronts, and a precipitation of 24 to 30 inches in 24 hours has been recorded at a few stations. Thunderstorms are of moderate frequency and come mostly in the winter.
The huge, volcanic domes on the island of Hawaii are higher than the depth of the trade winds, which are deflected around Mauna Kea and Mauna Loa, and the leeward slopes are large enough to permit the development of a local land and sea breeze. The sea breeze brings rain to the Kona district on the leeward side of Hawaii where a favored strip of land has been developed agriculturally between the elevations of 1000 and 2500 feet because of the ample rainfall there. Below 1000 feet it is too dry and above 2500 it is too wet for raising coffee and other specialty crops.