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Hawaii’s record 2018 rains may foretell wetter times ahead

Author: Matthew Cappucci / Source: Science News for Students

an artist's rendition of a rainy stormy sky, with two signs on a signpost in the foreground, one is a triangle with an exlamation mark in the middle, the other is a storm cloud with raindrops falling from it
Residents of Hawaii have survived several major deluges this year. And scientists say a warming climate may make such record-breakers ever more common.

One year ago, Hurricane Harvey shattered the U.S. record for most rain to come down in a single storm.

Last month, another hurricane dropped record rains, this time on Hawaii. Named Lane, its measured tally would seem to be the highest ever for this island state, and second nationally only to what Harvey unleashed on Texas.

The previous record for a tropical cyclone in Hawaii was measured at Kanalohuluhulu Ranger Station. That was during Hurricane Hiki in 1950.

The National Weather Service in Honolulu has now confirmed that Lane dropped 132.13 centimeters (52.02 inches) of rain between August 22 and 26. That total comes from an official government rain gauge on the Big Island (named Hawaii). “The previous record was 132.08 centimeters (52.00 inches),” the NWS reported in an August 27 statement. This, it concluded, shows that “Hurricane Lane has broken the Hawaii tropical cyclone storm-total rainfall record.”

However, NWS pointed out, this record will stand only “pending verification.” Confirming the feat requires a special probe. A meteorologist at the NWS forecast office said that could take months.

Why?

It’s due to how the hefty totals were measured.

NWS uses an automated “tipping bucket” rain gauge at all of its stations. The type used in Hawaii consists of a funnel that’s 30.5 centimeters (12 inches) in diameter. The funnel channels the incoming water into a bucket. That bucket tips over whenever 18.5 grams (0.65 ounce) of water collects in it. Each time the bucket tips, it registers a “click,” then adds another 0.0254 centimeter (0.01 inch) to its rainfall total.

This all seems pretty precise. But during high-end rain events, it’s not always an exact science. Sometimes, the rain comes down so fast that the bucket is constantly tipping. When this happens, the bucket can oscillate. Think of it as being a bit like a pendulum. When driven by some force, the bucket can pivot back and forth about a resting position. Here, the force is Earth’s gravity pulling on the bucket’s rainwater. The spring inside the tipping mechanism acts as a restoring force, pushing the bucket back toward its resting position.

Imagine you’re at some playground and a friend decides to ride the swing. She starts at the bottom. That’s the resting — or equilibrium — position for the swing. Now you give your friend a big push to get her going. The first few pushes are difficult as you drive the back-and-forth oscillation of the swing. But after a while, your friend swings forward, then backward, then forward and backward until each additional push requires relatively little effort. The driving force required to sustain the swing’s motion has become less.

The same thing happens in a rain gauge. As the rain falls super fast, the bucket is constantly swinging. That bucket now can become prone to tipping even when it’s not totally full. If it does, it will prematurely register a click. That will over-report the amount of rain that fell.

This will most likely occur during exceptionally heavy rains. Probing whether this likely occurred will take time.

In fact, meteorologists suspect that Hawaii’s record will stand. That’s because other stations in the area recorded similar values. And many of their rain gauges work differently. An official NWS gauge in Waiakea Uka registered 125.67 centimeters (49.48 inches). Another in Saddle Quarry saw 123.24 centimeters (48.52 inches). And a privately-owned instrument nearby was reportedly drenched with 149.35 centimeters (58.80 inches) of rain!

Hurricane Lane also snagged a spot as the second-biggest rain total in the United States since 1950. The only more dramatic U.S. rainmaker? Hurricane Harvey in August 2017. That storm spun for days over the…

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