The Big Island is not one climate. Hilo's gauges read 120 to 127 inches of rain a year. The leeward shoreline gauges at Kailua Kona and Puʻukoholā Heiau near Kawaihae read under ten in the same 1991 to 2020 normals. A statewide forecast tells you almost nothing about the beach you are standing on, and the wettest and driest months are not the same on the two coasts. Here is the month-by-month read, side by side.
Trade winds prevail 80 to 95 percent of the time from May through September and 50 to 80 percent from October through April. Where those winds hit the windward slopes, they make rain. Hilo and the Hāmākua and Puna coasts sit in that band and are wetter than anywhere else in the state.
The Kona coast does the opposite. It sits in the lee of Mauna Loa and Hualālai, and its summers are wetter than its winters. At Nāpoʻopoʻo the average monthly rainfall is 2.80 inches in winter against 3.70 in summer. That is the reverse of the usual Hawaiian pattern, and the rain there has a reliable time of day.
Hilo highs sit at 79°F with lows of 64 to 65. Rainfall runs about 9.3 inches in January, 9.6 in February and 13.4 in March. Kailua-Kona's coolest month is January, averaging about 71°F, and February is its driest month at roughly 1.6 inches. December and January are Kona's least cloudy months, at under 50 percent cover against an annual average of 57.
This is also when trade winds are least dependable, blowing 50 to 80 percent of the time from October through April. Major storms deposit a foot or more of snow on the upper slopes of Mauna Kea and Mauna Loa several times a year on average, almost always between October and May.
April is still wet in Hilo at about 11.5 inches, then May drops to about 8.1 and June to about 7.4. June is Hilo's driest month, which is worth knowing if the volcano and the east side are the point of your trip.
On the Kona side the trend runs the other way. April through June are the cloudiest months there, at roughly 70 percent cover, as the afternoon shower cycle sets in. Water temperature off Hilo climbs from its March low of about 75°F to about 78 by June.
These are the warmest months. Hilo highs reach 83°F with lows of 69 to 70. Kailua-Kona's warmest month is August at about 78°F. Kona's wettest months are July and September at about 2.9 inches each, still less than Hilo's driest month; Hilo itself runs about 10.8 inches in July, 9.9 in August and 9.9 in September.
The daily rhythm matters more than the monthly total on the leeward side. Kona skies are clearest from 8 to 10 a.m. and reach roughly 80 percent cloud cover by 2 p.m. Water temperature off Hilo peaks at about 80°F in September. Central Pacific hurricane season runs in the background from June 1 to November 30.
October holds summer's warmth on the east side, with Hilo highs still near 83°F and about 9.8 inches of rain. Then the windward side turns: November is Hilo's wettest month at about 15.5 inches, with December around 11.6. Fifteen inches in a month is real rain.
Kona goes the other way. November is one of its two driest months at about 1.7 inches, and December and January are its least cloudy at under 50 percent cover. Humpback whales start arriving in Hawaiian waters in November.
Coastal numbers do not apply once you go up. The Kīlauea summit area sits at roughly 4,000 feet, and the National Park Service says the weather there varies daily and may be rainy and chilly any time of year, running 12 to 15 degrees cooler than at sea level. Volcano Village sits in that same band, which is why people show up in beach clothes and regret it. Waimea, also well upslope on the north side, is likewise cooler than the coast below it.
The summits are another matter again. At high elevation the relative humidity is generally below 40 percent and often falls to 10 or even 5 percent; the Mauna Loa Observatory gauge averages just 14.3 inches of rain a year. Snow falls on the upper slopes several times a year, almost always between October and May.
The first is vog, the hazy air pollution caused by volcanic emissions from Kīlauea. Under trade winds the far downwind area is primarily the west side of Hawaiʻi Island, which gets mostly fine particles, while communities closer to the vents, from Ocean View to Hilo, get both sulfur dioxide gas and fine particles. The national park warns that hazardous volcanic gases can be a danger to sensitive groups and tells visitors to check air quality frequently.
The second is runoff. The Department of Health warns that after any storm event, storm water runoff may contain sewage, harmful micro-organisms and chemicals, and that the public should stay out of beaches or streams contaminated by brown water. Coastal water typically clears within about 48 hours through tidal flushing, but the guidance is to wait for further instructions from the Department of Health rather than to guess. Check live conditions before you get in.