Hilo (Big Island) weather in September

Hawaii Volcanoes-forum threads on vog describe it as forming when sulfur dioxide and other volcanic gases react with oxygen and moisture in sunlight, posters calling this a real, ongoing factor for anyone with asthma. Regulars call the Volcano area surprisingly often clear, prevailing trade winds generally pushing the haze toward Kona instead. Posters describe conditions as genuinely variable day to day, wind and weather both real factors in how much vog reaches a given trail. Regulars call checking air quality worthwhile for anyone sensitive, conditions capable of shifting within a day. Posters recommend flexible plans given the visitor center's construction schedule, renovation capable of affecting access as early as September. Regulars call the park still genuinely worth visiting regardless of vog, the volcanic landscape striking on a clear day. The record: 21.2C days feeling like 22C, 15.8C nights, an extraordinarily high 122.8mm across nearly 18 days and 81 percent humidity.

Regulars are specific this counts as one of the wetter, more humid months of the year here, June through November generally the most humid stretch, with heavy but short-lived showers typically arriving at night rather than disrupting the day. Our row backs that hot, genuinely wet read, 29C reading as a scorching 33C, over 21.4C nights feeling 23C, with a substantial 180.7mm of rain across 22.4 days and humidity at 83 percent. Regulars are consistent daytime temperatures in the 80s Fahrenheit feel noticeably hotter than the number alone suggests given the humidity, this side of the Big Island reliably wetter than the leeward Kona coast. The sea sits at a warm, swimmable 26.1C. The record: 12.2 hours of sun, a 19.1 kph wind and AQI 20.

Regulars name March one of this side of the island's two wettest months, alongside November, and explain the mechanism: trade winds carry moisture off the Pacific, and the clouds squeeze out rain as they climb the cooler slopes of the two big mountains, arriving as a squall that can turn a sunny minute into a downpour and back again within the hour. Our row backs the intensity, 249.4mm across 19.4 days at 85 percent humidity, alongside 26.9C reading as a real 30C over 18.9C nights. Their practical answer to a full washout is a short list of indoor stops - the Pacific Tsunami Museum, the Lyman Museum, Imiloa Astronomy Center - though most days the rain is the shower-then-sunshine kind that barely interrupts a plan. The sea holds 24.3C. The record: 10.3 hours of sun, a 17.8 kph wind and one of the cleanest air readings in this project, AQI 27.

Regulars here make two points that sound contradictory and are not. February is among the drier months on this coast - November is the wet one - and this coast takes two to three hundred inches a year. Both hold because of how it falls: a squall blows in, it pours, and shortly after it is sunny again, repeating through the day. Our row is that pattern in figures, 306.7mm across 16.4 days, nineteen millimetres each. They also warn against planning on it: dry spells of a week happen on the windward side, but nobody can say which winter month will get them. Days are 26.6C over 18.1C at 84 percent humidity. The record: 10.1 hours of sun, a 24.5C sea and AQI 32.

Hilo is the rainiest town in the United States, well over 200 inches a year, and locals still name May and June as the best time on this side of the island. Our row shows how both are true at once: 195mm across 22 precipitation days, and 11.7 hours of sun a day. The pattern they describe is squalls that blow in off the bay, dump hard and are gone, with a shower in the morning and another in the afternoon between stretches of sunshine - so the sky changes several times a day rather than settling. May is not the wettest month of the Hilo year. The island is two climates: rain on this side, dry over in Kohala, and a bad forecast here says nothing about there. The record: 28C over 20.1C reading as 31C, humidity 85 percent, a 24.9C sea and AQI 26.