Conditions on Hawaiʻi Island run from hot and dry on the Kona coast to wet on the Hilo side, cool and rainy at Kīlauea's summit near 4,000 feet, and below freezing on Mauna Kea in any month. Packing for one climate is how people end up cold at 9,200 feet and soaked at the volcano on the same trip. Pack for all four, and expect to carry layers rather than leave them in the car.
Two places on this island get genuinely cold. Mauna Kea's summit is 13,803 feet and the Visitor Information Station sits at 9,200 feet. The mountain's safety guidance states that winter conditions may occur at any time, causing freezing temperatures, 100-plus mph winds, and zero-visibility whiteouts. Hypothermia is a listed hazard, with instructions to be prepared for winter weather, use adequate cold-weather clothing, and limit your exposure to the cold. A hoodie is not that. Read the Mauna Kea guide before you go up.
The second is Kīlauea's summit area in Hawaiʻi Volcanoes National Park, around 4,000 feet, where the Park Service says temperatures may be 12 to 15 degrees cooler than at sea level and it can be rainy and chilly any time of the year.
The windward side, from Hilo through Hāmākua and down the Puna coast, is the wet side of the island, and passing showers are normal on otherwise sunny days. A light packable rain shell is worth the space. Quick-dry layers beat cotton once you are on a muddy trail.
The coastal end of the park, at the bottom of Chain of Craters Road, is the opposite. The Park Service describes it as often hot, dry, and windy with the possibility of passing showers. One park visit can go from cold rain at the caldera to open sun at the coast, so carry the layers with you instead of leaving them in the car at the trailhead.
The leeward Kona coast is dry and exposed. A wide-brim hat, sunglasses, and a rash guard or long-sleeve sun shirt cut down how much sunscreen you have to keep reapplying. Sunglasses matter at elevation too: intense solar radiation is a listed hazard on Mauna Kea, where the guidance is to protect yourself with appropriate clothing, sunscreen, and sunglasses.
Bring a refillable water bottle per person. The summit air on Mauna Kea is extremely dry, and the suggested intake there is about 16.9 fluid ounces (500 ml) per person per hour.
Hawaiʻi County Ordinance 22-81, from Bill 167, took effect December 1, 2022. It makes it unlawful to sell, offer for sale, or distribute for sale any non-mineral sunscreen in the county without a prescription issued by a licensed health care provider. Only titanium dioxide and zinc oxide are permitted as active ingredients. Unlike Maui County's ordinance, the Hawaiʻi County rule restricts sale rather than use, so a chemical sunscreen you brought from home is not prohibited.
Pack mineral sunscreen before you come. Do not plan on picking up your usual brand once you land, because it may not be on the shelf anywhere in the county.
Much of this island is rock. Beach entries at places like Hōnaunau Bay are lava ledge rather than sand, and park trails cross broken ʻaʻā and glassy pāhoehoe that will cut a bare foot and wear through a thin sole. Bring closed-toe shoes with real grip for trails, and water shoes or reef booties for rocky entries.
Rubber slippers are fine for town and for the sand at Hāpuna or Kua Bay. They are not fine on a lava trail or a wet rock ledge. Pack both, and accept that one pair of shoes will not cover this island.
Worth the space in the bag:
Skip the bulky gear you can rent locally. Do not skip the warm layer, which is the one thing you cannot improvise at 9,200 feet at 9 p.m.