The Rainiest Place On Earth

A massive warehouse facility in Tsukuba, Japan, equipped with 550 nozzles that can simulate rainfall intensity from 15 to 300 millimeters per hour.

The science conducted here keeps tens of millions of people safe by helping researchers study and prevent natural disasters.

Can simulate the most intense rainfall ever recorded—305 millimeters in one hour, which occurred in Holt, Missouri on June 22, 1947.

The facility allows researchers and visitors to experience and study extreme rainfall conditions in a controlled environment.

Yakushima Island near southern Japan is one of the wettest places on Earth, receiving up to 10 meters of rain annually—three times more than the Amazon rainforest.

Japan experiences massive typhoons that pick up energy from the Pacific Ocean and dump meters of rain on the islands.

Most rainfall evaporates or seeps into soil naturally, but too much rain in a short period—especially after typhoons—can lead to serious problems like flooding.

Scientists at NIED use the rainfall simulator to study these problems and develop prevention strategies.

Nozzles contain four holes of varying diameters, producing raindrops of different sizes that fall at different speeds.

• Gravity pulls raindrops down • Air resistance slows their descent • Larger droplets have higher weight-to-surface-area ratio and faster terminal velocity

A one-millimeter raindrop falls at two meters per second, while a three-millimeter raindrop falls at six meters per second.

Raindrops are nearly spherical but flatten on the bottom where they encounter air. Very large drops become parachute-shaped before breaking into smaller droplets.

Typhoon Prapiroon caused floods across Japan in July 2018, with some regions receiving nearly two meters of rain in just 10 days.

• More than 8 million people evacuated from homes • Over 200 people died • Property damage exceeded 1 trillion yen (nearly $10 billion)

The capital has over 100 rivers crisscrossing through a city of nearly 40 million people, making it very susceptible to flooding.

Pipes and tunnels under Tokyo lead to enormous water storage tanks. During Typhoon Hagibis in 2019, the system diverted 12 million cubic meters of water and prevented an estimated $1.7 billion in damage.

Japan's mountainous terrain with valleys, combined with heavy rainfall, creates perfect conditions for landslides. NIED has identified over 700,000 places where landslides pose significant threats.

• Slope angle • Amount of rainfall • Type of soil • Minerals present • Vegetation on the slope

Contrary to belief, water doesn't make soil slippery. Instead, water seeping into porous soil increases water pressure in pores, which decreases friction between grains and triggers slides.

Some areas sit on quick clay layers that lose structural integrity when exposed to intense rainfall, becoming liquid and causing catastrophic landslides.

• Steel beams and mesh to anchor slopes • Excavate top soil layers to decrease slope angle • Drill pipes into slopes to drain water

Create catchment zones and diversion channels—large holes that catch or divert landslides before they reach residential areas.

Trees are highly effective; their roots anchor slopes and their water uptake drains soil naturally. Deforestation in steep areas increases landslide frequency.

Over 85% of landslides in Seattle had at least partial human trigger from excavation or added weight. British Columbia has experienced a tenfold increase in landslides after deforestation for timber.

The simulator is used to test how drones fly in rainy and windy conditions.

Tests improve car hardware and software for detecting objects in rain conditions.

Both cameras and LIDAR sensors lose visibility and accuracy in rain, so testing in a controlled, repeatable environment helps engineers develop weather-resistant solutions.

The perfectly controllable environment allows systematic development of technologies that must operate safely in extreme weather.

Japan's rainfall patterns are shifting with more intense, shorter-duration storms becoming increasingly common.

• 50 millimeters per hour rain events have become 40% more common in the last 30 years • 100 millimeters per hour rain events have become 70% more common

Climate change will increase flooding rates and landslide occurrence in the coming decades.

Research at the world's largest rainfall simulator is becoming increasingly vital for preparing communities for future extreme weather events.