Big Island climates are fairly stable year-round. As a rule, it’s cooler as you move inland and up, and it’s wetter on the windward north and east coasts. The western Kona Coast is perennially sunny and usually hot, though go a few miles upland into coffee country and the air is considerably cooler.
On the eastern coast, Hilo is notoriously rainy, with an annual rainfall of over 100in. But temperatures remain balmy and, except for rainstorms, the typical drizzle is innocuous and short-lived. Good luck finding someone to predict the Windward Coast weather; even newspapers simply print endless variations on ‘sunny, with a chance of rain.’
At 4000ft, Hawaiʻi Volcanoes National Park (and the nearby town of Volcano) is cooler than Hilo, with chilly nights and occasional rainstorms. But the coolest town on the island is Waimea, where afternoon fog rolls across the grassy plains daily.
On the windward side of Mauna Kea, along the northeastern Hamakua Coast, around 300in of rain falls annually – but this water usually gets squeezed out on the mountain’s flanks, at around 2500ft, and the coast and summit are much clearer. In fact, only about 15in of precipitation falls on the summits of Mauna Kea and Mauna Loa, mostly as winter snow that caps these peaks with white.
In January the average daily high temperature is 65°F at Hawai’i Volcanoes National Park, 79°F in Hilo and a toasty 81°F in Kailua-Kona. August temperatures rise only 5°F or so. Nighttime lows are about 15°F less.
The National Weather Service Hilo provides recorded forecasts for the Big Island (961-5582) and for island water conditions (935-9883). Hawaiʻi Volcanoes National Park (985-6000) has recorded information on eruption activity and viewing points.