Hawaii has two seasons, winter and summer. The main differences are the amount of rain and much less so the temperatures, especially from place to place.
The average temperature year-round is 75º F between the warmest weather in August and the coldest weather in January. The daytime temperature may vary 5 – 7º F.
February is the most unpredictable month. Temperatures depend on elevation. In summer, go over 2,000-ft and it’s definitely cooler; temperature drops three degrees for every 1000-ft of elevation. But it also rains more in the mountains and upcountry, especially on the windward side. The weather in the off-season travel period (April through December 15, and especially April to mid-July and September to Christmas) may actually be better than the weather during high-season.
However, it can rain hard and long in winter. Even on the dry Kohala coast on the Big Island, it can rain on and off for weeks in January. In the Hilo area, over 210 inches of rain fell in 1990, where it can deluge an inch an hour. Usually it’s possible to find at least partly clear skies somewhere on an island even when it’s raining somewhere else. Frequently you’ll see rainbows along with rain or mist.
Starting with summer, from May through mid-October, daytime temperatures are in the 80s and night times from the low 70s to the low 80s. Consistent trade winds keep summer temperatures tolerable. Konas can blow in from the south or west and bring humid, sticky, or sweltering weather and thunderstorms. Showers are short. Days are longer.
Winter runs from mid-October through April. During this period, rains are more frequent, longer and more intense, even on leeward coasts. Daytime temperatures at sea level are in the mid-70s to low 80s.
Nighttime temperatures run from the low 60s to the low 70s. Trade winds that usually blow in from the northeast are somewhat erratic and in February, may actually cease for a few weeks.