Weather in The Hawaiian Islands is very consistent, with only minor changes in temperature throughout the year. There are really only two seasons in Hawaii: summer (kau) from May to October and winter (hooilo) from November to April. The average daytime summer temperature at sea level is 85° F (29.4° C), while the average daytime winter temperature is 78° (25.6° C). Temperatures at night are approximately 10° F lower than the daytime.
The islands are an incredible collection of diverse micro-environments, each with its own unique weather plants and animals. As a result of the shielding effect of volcanic mountains and the differences in weather at various elevations, you can find tropical rainforests, cool alpine regions, arid deserts and sunny beaches—all within the span of just a few miles.
Throughout the year, Hawaiian weather patterns are affected primarily by high-pressure zones in the north Pacific that pump cool, moist trade winds down onto the islands’ northeastern slopes. These winds are forced upslope by the mountain heights, where their moisture condenses into clouds that produce rain. Most of the rain falls in the mountains and valleys on the windward (northeastern) side of the islands. It is this weather phenomenon that creates Hawaii’s rich, green, tropical environment.
The wettest months are from November to March, but winter rains do not generally disrupt vacation plans, since the weather is very localized. This means that if it is raining where you are, there is almost always a sunny spot to be found around the coast.
The Hawaiian Islands’ trade winds mean there is almost always a cooling breeze here. Several times during the year the trade winds will stop completely, and the wind will switch around to come out of the south or west, bringing stormy or hot, sticky weather. Islanders sometimes call this kona weather, because kona means leeward or south, and this points to the direction from which these weather systems arrive.
The Hawaiian Islands’ near-shore water temperatures remain comfortable throughout the year. The average water temperature is 74° F (23.3° C), with a summer high of 80° F (26.7° C). Wave action varies dramatically between island coasts and seasons. Summer waters are typically gentle on all beaches. During the winter on many north shore beaches, Pacific storms drive ocean swells towards the islands, creating The Hawaiian Islands’ legendary big waves.
Wave conditions are often very localized, so if the waves are too big on your beach, you can usually find calmer water at a more sheltered beach. Strong currents can make any beach unsafe at any time during the year, particularly in the winter. Ask your hotel staff or a lifeguard about ocean currents or look for warning flags and posted beach conditions.
Please heed all weather warnings before hiking, swimming, sailing or participating in any outdoor activities.Many visitors are drawn to the natural beauty found in higher elevations such as Kokee on Kauai, Haleakala on Maui or Kilauea on the island of Hawaii. Temperatures in these higher locations drop 3.5° F for every 1,000 feet above sea level that you climb, so dress appropriately with pants and several layers of clothing. At an elevation of 10,023 feet, the summit of Haleakala can be as much as 30° F cooler than resort areas on the coast.
Also note that because of these high elevations, there is less protection from the sun's powerful UV rays, so come prepared with sunblock and sunglasses.
Vog is the local term for "volcanic fog" and it describes the hazy air pollution that occasionally hangs over the islands. Vog is caused when sulfur dioxide and other gasses from Kilauea's Halemaumau Crater (on the island of Hawaii) mix with moisture in the air and sunlight. Under extreme conditions—when the volcano is active and the winds carry the fumes north to the rest of the island chain—vog can be hazardous to plants, animals and humans. The most common effects are headaches, watery eyes and breathing difficulties. These effects can be especially pronounced in people with respiratory conditions and young children. It is not advisable to exercise or participate in strenuous outdoor activities when the vog is very heavy.