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At least <a href="https://www.washingtonpost.com/world/2021/07/21/china-floods-zhengzhou-volunteers/" target=_blank>25 people were killed</a> in subway cars amid devastating flooding in eastern China on Tuesday. Zhengzhou, the capital of Henan province and home to more than 10 million people, suffered an extreme downpour that proved the heaviest ever observed in China and among the most significant on record globally.

A staggering 7.95 inches (201.9 millimeters) of rain came down between 4 and 5 p.m. Tuesday, one of many reports of significant to prolific rainfall that resulted in deaths and damage across eastern China. It contributed to a daily rain total that exceeded 24 inches in Zhengzhou.

That is almost a year’s worth of precipitation; the <a href="https://twitter.com/Minghao_Zhou/status/1417726094648696832" target=_blank>city averages 25.4 inches annually. </a>

Passengers died when their train cars filled with water, the subway system suddenly overwhelmed by rapid torrents of rainfall gushing in. Dramatic <a href="https://twitter.com/NewsForAllUK/status/1417533544906248192" target=_blank>footage posted to social media</a> captured a woman being rescued from a whirlpool, presumably leading to a drain or cistern. Abandoned vehicles<a href="https://twitter.com/UNFCCC/status/1417766452443164675" target=_blank> littered flooded roadways</a>; others were simply submerged or <a href="https://twitter.com/RitaBai/status/1417491524972122113" target=_blank>swept away. </a>

Rescue efforts underway after Chinese floods displace 1.2 million

The calamitous rainfall resulted from a trifecta of meteorological conditions that, intensified by human-caused climate change, resulted in catastrophe.

Tropical moisture had been in place across much of southern China. Typhoon Cempaka made landfall in Yangjian, west of Hong Kong, on Tuesday morning at Category 1-equivalent strength. A second typhoon, In-fa, remains a high-end Category 2 that’s churning north of Taiwan. The former helped pump in moisture from the South China Sea, while the latter has been inducing a moist onshore flow from the East China Sea.

Meteorologists refer to precipitable water indexes (PWATs), or the total amount of moisture present in a column of air, as an indicator for heavy rain potential. PWATs over 1.5 inches can support heavy downpours; 2-inch PWATs are tropical. On Tuesday, PWATs in eastern China reached a whopping 2.5 to 3 inches.

That had two effects. The first was, obviously, providing the moisture necessary for heavy downpours. Much of that moisture pooled along a diffuse, semi-stationary boundary known as the Meiyu front. It passes through eastern China and north of Taiwan during the summertime, helping to focus heavy downpours.

The other effect of the high PWATs was increased precipitation efficiency. When rain, hail or snow, collectively known as “hydrometeors,” fall to the ground, the air below is seldom at 100 percent relative humidity. As a result, dry air induces some evaporation on the hydrometeor’s way down, eroding its mass from the outside. It’s not unusual for a raindrop to lose 30 or 40 percent of its water on the way down.

China Daily

But with the entire column saturated, meaning the air was considerably moist, there wasn’t much dry air to eat away at the raindrops. That meant most of the water in the clouds was falling all the way to the surface, stacking up more quickly.

Those elements helped the downpour to kick into overdrive. By the time the worst began mid- to late afternoon, morning rains — also tallying about 8 inches — had left the ground saturated. Many drainage systems were already close to maximum capacity before the worst rolled around.

Altogether, the dice were loaded for a top-tier event. Doppler radar data showed that the thunderstorm cell that produced the downpour sat and hovered over Zhengzhou. Weak upper-level winds, typical of a summertime weather pattern in mainland China, meant the storm was in no hurry to move.

Inside the flooded China subway, which trapped commuters up to their necks in water

Even after the heaviest bouts concluded, another 6 to 8 inches of rain fell across the city.

In addition to setting a record for China, the remarkable rainfall is in elite company when compared against similar records around the world. According to the World Meteorological Organization, the greatest one-hour rainfall reliably observed globally was 305 millimeters, or 1 foot, in Holt, Mo., on <a href="https://wmo.asu.edu/content/world-greatest-sixty-minute-one-hour-rainfall" target=_blank>June 22, 1947. </a>

The Chinese national record in Zhengzhou may be the most prolific short-term rain event to ever affect a major international city.

Precipitation extremes like this may become more common in the future as the world continues to warm thanks to climate change. Warm air can hold more moisture, which translates to more “moisture loading” of thunderstorms and a greater propensity for extreme downpours. It’s a phenomenon that’s already been observed in many major U.S. cities, <a href="https://www.washingtonpost.com/weather/2018/12/10/houstons-latest-deluge-fits-into-alarming-trend-toward-more-extreme-rainfall/" target="_blank">including Houston, where high-end rainfall events are twice as common as in the 1970s. </a>

Perspective: Fire, flood and heat disasters are an indictment of decades of poor planning

Harvard University, B.A. Atmospheric Sciences

Matthew Cappucci is a meteorologist for Capital Weather Gang. He earned a B.A. in atmospheric sciences from Harvard University in 2019, and has contributed to The Washington Post since he was 18. He is an avid storm chaser and adventurer, and covers all types of weather, climate science, and astronomy.

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