Half of the World's Rain Falls In Just a Dozen Days a Year

Illustration for article titled Half of the Worlds Rain Falls In Just a Dozen Days a Year
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When it rains, it pours. Tinder dates, job offers, and bad news all abide by this maxim. But it turns out that precipitation itself does, too.

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Heavy downpours have been on the rise thanks to climate change. But a new study in Geophysical Research Letters puts why that matters in context. It turns out that on average, half the world’s rain falls in just 12 days a year.

This doesn’t mean that there are just 12 days where the entire planet gets hammered with rain and snow a la The Day After Tomorrow. Rather it’s that many places get half their yearly rain over their 12 wettest days. And those wettest days are getting wetter, meaning in the future bouts of raininess are likely to become even more concentrated.

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If the fact that it takes just 3 percent of the year for half the world’s rain to fall shocks you, you’re not alone. The scientists who wrote the study were astounded, too. Angeline Pendergrass, a climate scientist at the National Center for Atmospheric Research, told Earther she had toying with the question of quantifying how long it takes for the planet’s rain to pile up. After some tinkering, she worked up a rough plot and showed it to a few other researchers during talks she gave.

“Everyone I showed it to was really surprised at how large a contribution extreme precipitation was to total precipitation,” she said. “I was surprised, too.”

And thus, a study was born. First, Pendergrass and a colleague gathered rainfall data from a variety of sources. That included high quality weather station data for most of the world from 1999-2014 as well satellite data that provides an approximation of precipitation around the globe, including areas with a paucity of stations like Africa and South America. They also used climate models to project precipitation changes by century’s end.

The results show that a quarter of all precipitation falls in six days, half in 12 days and a whopping three-quarters of it falls in 27 days.

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The results vary around the world. The dry nature of the Southwest or Western Australia means in those places, wet days can pile up an even bigger chunk of the year’s precipitation. In places that are uniformly wet like the UK or the tropics, it takes more days to accumulate a half year’s worth of rain.

Climate change is likely to amplify the extremes even further as it combines to make heavy downpours more common and intense, while simultaneously pushing wet places to be wetter and dry places to be drier. By the end of the century, the study shows those trends will shave a day off the half year of rain mark, lowering it from 12 days to 11 days.

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“It’s mindbendy that what ends up happening with distribution of precipitation in response to warming is you have more floods and more drought,” Pendergrass said.

Indeed, the impacts of changing precipitation patterns are far-ranging and potentially severe. In a world of increasing weather whiplash, reservoirs will have to be able to take up and safely store rainfall in bigger gulps, farmers will have to prepare for drought as well as getting their fields flooded, and cities will have to reconfigure their sewers, bridges, transit system, and well, basically everything.

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Managing editor, Earther

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DISCUSSION

dnapl
Dense non aqueous phase liquid

Woohoo. Earther reviewed a paper that is open to all. Good stuff, Brian.

I read skimmed it and couldn’t figure out if precipitation data was for land only. Kinda a good question considering earth surface is about 70 odd percent not land. From data and methods section of the paper:

We choose stations from the Global Climate Observing System [GCOS] Surface Network , which have higher quality than the full data set and are distributed globally, though concentrated in North America, Eurasia, and Australia. We include only stations with sufficient temporal coverage: at least 70% of data available for at least 70% of years. For seasonal calculations, we combine each December with the following January and February, and we require data to be present for 70% of days during the season (June, July, and August [JJA] and December, January, and February [DJF]).

There are ocean sites in GCOS network, but it’s not stated if data from those stations was used.

Rain is a giant heat and mass transfer mechanism between atmosphere and ocean. It better get understood and fast or we’ll get those climate kids to sue climate scientists’ asses off. Just like the geophysicists got sued in Italy after an earthquake. 😉, as the kids these days say.