Why the Big Bog in East Maui Is Now Hawai‘i’s Rainiest Spot

The state climatologist explains where the wettest places are and what factors contribute to all that rain.

September 27, 2023

Tori DeJournett,

09 23 Fob 1800x1200 Rainiest Places 1 — The Big Bog, the rainiest spot in Hawai‘i, sits at an elevation of 5,400 feet, where it catches the prevailing northeast-east trade winds as they rise up the windward slope of Haleakalā and drop their moisture. | Photo: courtesy of Pao-Shin Chu

Many places in Hawai‘i get lots of rain, especially in the winter, but the wettest location in the Islands is a section of the Hāna Forest Reserve in East Maui called the Big Bog.

“The east side of Maui is aligned in a northwest-southeast direction, in a way almost perpendicular to the prevailing northeast-east trade winds,” says Pao-Shin Chu, a professor of meteorology at UH Mānoa and Hawai‘i’s state climatologist.

He says that as cumulus cloud clusters approach the windward slope of Haleakalā, “they are forced to rise and produce cooling, saturation, clouds and rains.” It’s called an orographic rainfall effect, which happens when air masses flow over high topography.

The Big Bog, at an elevation of 5,400 feet, experiences an average of 31.6 feet of rainfall a year. Chu says that average annual total, and all the rainfall figures in this story, are based on various data sources covering 70 years, from 1950 to 2019.

The second wettest spots in Hawai‘i are Kawaikini and Wai‘ale‘ale, mountain summits in central Kaua‘i that are near each other. Chu says both get 29.8 feet of rain a year.

Kaua‘i is the northernmost island in the Hawaiian chain, which means “it is more susceptible to midlatitude weather systems such as a cold front, Kona storm or upper-level low than other islands,” Chu says.

And because of Kaua‘i’s round shape, the island is vulnerable to weather systems from any direction. The island is also below the trade wind inversion.

That results in the island being “subjected to weather systems from all directions – from north, from south, from east, from west – and they all converge into the center of Kaua‘i, so it makes this area very wet,” Chu says.

Wet Spots on Hawai’i Island and O’ahu

Hawai‘i Island’s wettest spot is in the Hilo Watershed Reserve, which has an average of 23.6 feet of annual rainfall. The Wailuku River begins within the reserve and feeds numerous waterfalls, including the popular tourist attraction Waiānuenue (Rainbow Falls).

Hawai‘i Island is home to two tall mountains, Maunaloa and Maunakea, which create lots of orographic rainfall effects.

O‘ahu’s wettest spot is in the ‘Ewa Forest Reserve, on the windward slope of the Ko‘olau range near Punalu‘u.

Although O‘ahu has a similar orientation as East Maui, “the Ko‘olau Range is not as tall and massive as Haleakalā, so the interaction between trade winds and island topography is perhaps less pronounced as Big Bog on Maui,” Chu says. Nonetheless, that wet spot in the ‘Ewa Forest Reserve gets an average annual rainfall of 21.2 feet.

Chu also listed the wettest towns on each of the four most populous islands and ranked them by their mean annual rainfall from 1951 to 2022.