Big Bog, Maui

The Big Bog is a high-altitude bog on the island of Maui, the largest in the Hawaiian Islands. It is located on Haleakala's east rift zone, at the border between Hāna Forest Reserve and Haleakalā National Park. It is alleged to be one of the wettest places on Earth, with a reported annual rainfall of 404 in for the period 1992-2018 according to the University of Hawaiʻi at Mānoa.

While the summit of Mount Waiʻaleʻale on Kauai has long been considered the wettest place in the Hawaiian Islands, and was claimed to be the second wettest place on Earth, its NOAA-reported annual rainfall of 373.85 in is exceeded by Big Bog's 30-year average.

Climate
The Big Bog has a tropical rainforest climate (Köppen Af), with no observable dry season and nearly constant torrential rainfall. Prior to the establishment of the station there in 1992, rainfall for Big Bog was estimated at around 4600 mm per year. However, the first full year of recorded data showed 13995 mm of rainfall, which is one of the highest annual rainfall totals measured in the Hawaiian Islands. Since then, the annual average has been recorded as 404 in. Clear days are essentially nonexistent, and even when it is not raining, it is almost certainly cloudy or foggy. The lack of adequate drainage has caused moisture to accumulate, forming the bog.

Causes
The Big Bog lies at 5400 ft, very close to the trade wind inversion layer, leading to persistent transport of moisture rich air by the northeast trade winds up the steep mountain slopes. These trade winds condense to form clouds and precipitation. Its reputation as the cloudiest place in the Hawaiian Islands is supported by its average solar radiation and potential evapotranspiration being the lowest amongst recorded locations in the state, and relative humidity and cloud attenuation the highest.