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Think it’s wet in New Zealand? Try 331 days with rain in the USA

A week in a row with rainy conditions is bad enough, but could you imagine being in a location where measurable rain has occurred hundreds of days in a row?

Windward-facing locations of Hawaii are one of the United States’ prime locations for persistent rainfall day-after-day, and where the record for most consecutive days with measurable rain is held.  From 1939-40, Maunawili Ranch on the island of Oahu saw 331 straight days with measurable rain, according to Brian Brettschneider‏, a climatologist with the University of Alaska-Fairbanks and the Western Regional Climate Center (WRCC).

In meteorological records, measurable precipitation is defined as 0.25mm or more of rain and/or melted snow on a given day. 

The top five locations that have seen the most number of days with measurable rain in a row. (Source: Western Regional Climate Center/GHCN-D)

Four other locations in Hawaii round out the top five for the most consecutive days with measurable rain in the United States: two on the Big Island and two on Oahu. All those locations have seen streaks with more than 200 days of measurable rain. The most recent entry on the top five list is Pali Golf Course on Oahu where 247 days in a row with measurable rain was recorded 1993-94.

Another incredible rainfall record in Hawaii is a location on the Big Island that once saw 881 straight days with at least a trace of rain. A trace is a small quantity of precipitation that can’t be measured.

Brettschneider compiled the information on the Hawaiian rainfall streaks from the Global Historical Climatology Network – Daily (GHCN-D) and said the streaks were previously unidentified records.

In the Lower 48, the longest stretch any location has seen measurable precipitation (rain/snow) is 79 days near Otis, Oregon, in the winter of 1997-98. Alaska’s record of 88 consecutive days with measurable precipitation was set in Ketchikan in 1920.

Hawaii’s Rainfall Patterns Explained

Hawaii’s mountainous terrain plays a large role in its rainfall patterns.

The wettest locations are on the windward side of the island chain where moisture provided by trade winds slams into the mountains causing rainfall to be enhanced. On the leeward side of the islands, much drier conditions exist.

You can see this pattern on the map below with elevated windward areas shaded blue receiving more than 4.3 metres of rain annually. Some locations are estimated to have annual rainfall that exceeds 7.6 metres. Conversely, the dark red colored leeward locations see 200 to 760mm of rain annually. [It’s worth noting Fiordland in New Zealand can have over 14 metres of rain per year and is one of the wettest places on earth]

Average annual precipitation in Hawaii in millimeters.

Rainfall amounts in the mountains are further enhanced by “fog drip at higher elevations where the montane forest canopy intercepts cloud water,” the United States Geological Survey says.

The greatest precipitation totals ever recorded in both one month and one year in the United States occurred in Puu Kukui on the island of Maui, which is located in a favorable windward mountain location.

That site set the monthly record with 2.7 metres of rain in March 1942. The annual rainfall record set there was 17.9 metres in 1982.

Average annual rainfall quickly tapers off from 9.4 metres (9398mm) at Puu Kukui to less than 760mm near the lower elevations at the coast, which reinforces that elevation plays a huge role in enhancing rainfall at this site.

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