I know this is a concept that is probably foreign to the younger folks, but back in the year 19-something-something, my parents bought these large books from Reader’s Digest covering life and the various physical features of our planet. I remember reading in one of the books that the wettest spot on Earth was Waiʻaleʻale on Kauaʻi. Wow! The wettest spot on our planet was in our state. That was cool! The value of 440 inches per year sticks out in my head but who knows what the actual number was in the book. The entry in Wikipedia says 450 to 460 inches per year, depending on the source.
Kauaʻi on a rare cloudless morning. Kawaikini is the highest point on the island but Waiʻaleʻale just to the north of it is the wettest. I don’t know what the percentage of days in the year are this clear over Kauaʻi but my guess is that it’s in the low single digits. Photo credit: Elton Ushio. Annotations are mine.
At some point, I think while in college, I heard that the title of wettest spot in the world was taken by Cherrapunji in eastern India. The title was subsequently taken by Mawsynram, which is also in eastern India (see map below). Mawsynram receives the bulk of its rainfall during the summer monsoon season. According to the World Meteorological Organization’s (WMO) table of records, Mawsynram averages 467 inches of rainfall per year.
Well, at least Waiʻaleʻale was still the wettest spot in the United States! Then in 2013, the Bulletin of the American Meteorological Society published an article on the new online Rainfall Atlas of Hawaiʻi (Giambelluca et al, 2013). The article said that Big Bog in east Maui had an average annual rainfall of 404 inches. At that point in time, my data from the U.S. Geological Survey (USGS) had a 30-year running average for Waiʻaleʻale at just under 370 inches per year, which meant that Waiʻaleʻale was not even the wettest spot in the Hawaiian Islands! What happened to “450 to 460 inches”? Good question. Sources cited in the Wikipedia article (linked above) indicated a 1912 to 1945 data period for the 460 inches per year average. Assuming the data are correct, that’s a heck of a decrease!
Location of Mawsynram, the wettest spot in the world according to the WMO.
Location of Waiʻaleʻale on Kauaʻi. The rainfall data and gage location are from the U.S. Geological Survey.
Location of Big Bog on Maui: The gage location is from the Hawaiʻi Climate Data Portal.
The USGS operates an automated rain gage at Waiʻaleʻale and has a continuous record going back to 1968. Prior to that year, the record has significant gaps. Using the continuous record, I plotted the annual rainfall totals from 1968 through 2025 on the graph below. The overall trendline (dashed black line) and running 30-year average (dashed red line) are also included. While there is year-to-year variability, it’s evident that there has been an overall decline in rainfall since 1968. The running 30-year average has dropped from 406.03 inches in 1997 (first available year) to 355.31 inches in 2025. The downward trend also passes the Mann-Kendall trend test (using the method at real-statistics.com) at the 0.05 significant level. Looking at it another way, prior to 1995 there are 13 years with annual rainfall totals more than 400 inches. Since 1995, only three years have annual totals more than 400 inches.
Plot of annual rainfall totals for Waiʻaleʻale from 1968 through 2025. The red dashed line is a trendline based on the running 30-year average of the totals. The black dashed line is the linear trendline over the whole time series. The rainfall data are from the U.S. Geological Survey.
There are several possible reasons for this decrease in rainfall. One could be a change in the height of the trade wind inversion. A study by Cao et al. (2007) on the trade wind inversion at Līhuʻe and Hilo found a “weakly significant downward trend at Līhuʻe”. Why is this important? The height of the trade wind inversion marks the base of a stable layer where the air temperature temporarily increases with height instead of decreasing. This stable layer caps the vertical development of clouds, which, in turn, limits rainfall potential. While there are many factors involved in precipitation generation, shorter clouds generally produce less rainfall. Since Cao et al. found the inversion present roughly 82% of the time at Līhuʻe, it’s an important feature in determining annual rainfall.
Another factor could be changes in the lifted condensation level (LCL), which marks the bases of clouds. If the LCL trends higher over time, this could also produce a smaller cloud, especially if it’s in conjunction with a lowering trade wind inversion, which would also reduce rainfall potential. I don’t know if there are any LCL trends in Hawaiʻi, but it could be a possible reason. Changes in trade wind frequency, direction, and intensity could also play a role by changing the way terrain interactions with the low level winds force cloud and rainfall development. Garza et al. (2012) did find statistically significant changes in all three factors over time over Hawaiʻi, but their study did not extend to rainfall impacts. As is often the case, more questions than definitive answers!
To close out, it appears that the 450 to 460 average annual rainfall value at Waiʻaleʻale is badly out of date. Like maybe a hundred years out of date. But while it is no longer the wettest spot on Earth, you can definitely say it is one of the wettest spots on Earth (along with Big Bog, of course)! Oh, and one more thing. I want to give a big ‘ol shout-out to the USGS Pacific Water Science Center for all their efforts in operating and maintaining the Waiʻaleʻale rain gage. The site is very difficult to get to, and having kept a continuous record for so many decades is an amazing feat!
References
Cao, G., T. W. Giambelluca, D. E. Stevens, and T. A. Schroeder, 2007: Inversion variability in the Hawaiian trade wind regime. J. Climate, 20, 1145-1160.
Garza, J. A., P.-S. Chu, C. W. Norton, and T. A. Schroeder, 2012: Changes of the prevailing trade winds over the islands of Hawaiʻi and the North Pacific. J. Geophys. Res. Atmos., 117, D11109.
Giambelluca, T. W., Q. Chen, A. G. Frazier, J. P. Price, Y.-L. Chen, P.-S. Chu, J. K. Eischeid, and D. M. Delparte: Online rainfall atlas of Hawaiʻi. Bull. Amer. Meteor. Soc., 94, 313-316.