Friday, October 2, 2009

CoCoRaHS Important To Hawaii

On the island of Kaua`i, Mount Wai`ale`ale (“rippling water”) is purported to be the wettest spot on earth. This eroded volcanic core rises nearly vertically to just shy of a mile above sea level.

Rainfall records at the summit of Wai`ale`ale have been kept intermittently since 1910. The average rainfall since 1910 is 426 inches of rain a year, with 680 inches being the greatest annual rainfall recorded there (1982).

You’d need a 57-foot tall rain gauge that year to collect the entire year’s catch before emptying the gauge. Meanwhile the great sand dunes at Polehale on the leeward side of Kaua`i get an annual average rainfall of only 8 inches.

On O`ahu, the official 30-year average annual rainfall for Honolulu (on the leeward side) is 18.29 inches. On the wet, windward side of O`ahu, the tradewinds bring 60 to 280 inches to the mountain ridges.

On the big island of Hawai`i, rainfall on the east slopes of Mauna Kea can be nearly 300 inches, while on the west slopes, rainfall is unreliable and less than 10 inches per year. During a three-day rainfall in 2008, Hilo recorded just under 40 inches of precipitation.

Most rain comes in the form of isolated showers that come and go in a few minutes. When major storms bring heavy rain to Hawai`i, they’re pervasive and easy to identify.

But, the rainfall amounts vary greatly by locality.

On any given day, you can be standing in the sun while across the street your neighbor is getting a shower. When heavy rain hits, rapidly changing elevation, air currents, land formations, and other factors can mean that the amount of rainfall varies incredibly from place to place, even between localities that aren’t widely separated.

At the same time, its hard to get excited about reading the rain gauge on the dry sides of the island when it’s dry for weeks or months at a time. But when it rains, the effects on vegetation, stream beds, reefs and wildlife are so dramatic that the rain report is vital data.

Let’s proudly report those zeroes!

Thanks to Ben Black for the great entry.


  1. Beautiful Picture of the Water Fall. Super Amazing Rainfall Amounts in such short distances. Excellent story.

  2. I did a quick rain-year check, 10/01/2008 to 10/01/2009 to compare Hawaii to where I live in Oregon. This is the 08/09 totals:

    Precip. 51.88"
    Snow 38.8"
    Snow water 5.42"

    Wet but nothing compared to places in Hawaii...Great story and picture!!