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Weather

The short of it

The Big Island is:

 Rainiest on its north side, which boasts rainforests. Waipio and Waimanu valleys have rainforest conditions, though agriculture has displaced most of the native rainforest vegetation on the valleys’ floors.

 Less rainy on its east side, but still rainy enough to support lush tropical growth. Hilo’s streets, parks, and yards are bursting with flowers. A rainforest flourishes on the northeast slope of Kilauea volcano. Just think how much rain it must take to nourish a rainforest on the rim of the world’s most active volcano!

 Driest and hottest on the south and west. These areas lie in the rain shadows of the island’s volcanoes. While a rainforest grows on Kilauea’s northeast slope, a desert stretches down its southwest slope.

 Rainier in the mountains on any side of the island. However, the summits of Mauna Loa and Mauna Kea are frigid alpine deserts. At nearly 14,000 feet, altitude, not latitude, governs the climate of those summits.

The dry Kona and north Kohala coasts attract the most visitors, have the principal resorts, and have the most popular beaches. The figure below summarizes the situation:

The long of it

Hawaii’s coastal weather is temperate to a degree that puts the so-called “temperate” zones of the world to shame. The humidity is moderate, too: 50 to 60%, not the sweltering horror of some other tropical lands. It is warmer in the summer and cooler in the winter, but the “extremes” are only a few degrees apart—nothing like those on the mainland. (Hawaii’s mountainous interior, however, is quite another matter.)

Hawaii’s mild climate is determined largely by its tropical location and also by the northeast trade winds that sweep across it. The northeast trade winds—so-called because sea captains took advantage of them on their trade routes—are dependable, steady winds that blow from the northeast across the thousands of miles of open sea that separate the Hawaiian Islands from the continents. They are responsible for keeping the temperature and the humidity moderate. Since they are the prevailing winds in this area, the side of the island that faces them is called the “windward” side. The opposite side of the island is the opposite of windward; in nautical terms, “leeward.”

Sometimes the trade winds fail and are replaced by “kona” winds coming from the south. “Kona” means “leeward,” because it’s the leeward side of the island that more or less faces these occasional winds. Kona winds bring hot, sticky air. Fortunately, they are rare in summer, when they would be really unpleasant, and occur mostly in winter, when the lower overall temperatures moderate their effect. Kona storms are subtropical low-pressure systems that occur in winter, move in from the south, and can cause serious damage. There is apparently no pattern to them; in some years, they do not occur at all, but in others they occur every few weeks.

On the island of Hawaii, average temperatures in Hilo range from highs of 79-82°F to lows of 61-70°F. In Kona, temperatures range from highs of 80-82°F to lows of 62-68°F. The “cooler” ones are winter temperatures, the warmer ones summer. It’s rainier from November through March than it is the rest of the year. Hilo gets an average of 133 inches of rain per year, while coastal Kona may get as little as 15 inches of rain per year. Expect cooler temperatures, more wind, and considerably more rain if you are in a mountainous region. For example, the uplands of south Kona are rainy enough to support the nation’s only coffee plantations! The summits of Mauna Loa and Mauna Kea are often chilly, even by day, and can be bitterly cold at any time. Both are snow-covered in winter. Kilauea’s northeast side can be very cold because of its high rainfall, though its summit is only 4,078 feet high.