Friday, April 3, 2015

Atacama Floods

Dear CBS,

The thunderstorm wasn't the worst storm to hit Chile in seven years, it was the ONLY real rainfall to hit northern Chile in over a hundred year. You do know it's the Atacama Desert, the driest place on earth?

The last recorded rainfall was about 80 years ago. According to the Chilean deputy interior minister, Mahmud Aleuy, it's also “the worst rain disaster to fall on the north in 80 years.”

But any rain in the Atacama region is a disaster waiting to happen. When the rain falls, it has nowhere to go. No vegetation, no soil. Nothing to absorb rain. So, it's all runoff. Rain never ever falls on the Atacama. Not even on the plain.

OK, so there was a freak snowstorm in 2011. Now that was an anomaly. It was meant for the southern Andes but the high pressure marine layer was disrupted. And it snowed and snowed. No flash floods were recorded but no one knew how to drive in snow.

CBS reported that thunderstorms dumped the equivalent of 7 years' worth of rain in 12 hours. That's technically true. But how you presented the facts was a bit skewed. How much rain? Seven years' worth of rain is 24 millimeters which is 0.94488189 inches. OK, so that's almost an inch of rain (divided by seven years), not a lot of rain, but it had nowhere to go, and the desert certainly wasn't going to soak up that weird, alien stuff. Think of thick dust on on pavement. Now add water. what happens next? Flash floods.

Normal precipitation (in the from of fog) in the Atacama is about 0.07 inches (1.7 mm) a year. And the southern region's been experiencing a huge drought. But if normal is 0.07", then I guess it's been about 0.1" moisture a year in the Atacama for quite some while. 

The Atacama Desert, the driest place on Earth, has been deluged by torrential rains and floods have caused at least two deaths, swept away homes and left the region without power or infrastructure. So, yes, horrific flash floods were in order. And sadly lives and homes were lost. Seven rivers, including the Copiapo River all leapt their ancient banks, such as they were. The rivers don't get much of a workout as most of the Andes rain and snowmelt runs east to the Amazon.

What's weird about this weather is that virtually no rain ever falls this time of year. If it ever falls at all. That's the real story. The sudden autumn downpour on the heels of an unusually hot, dry summer in the throes of an eight-year drought, has left parched fields. A disaster waiting to happen.

Northern Chile is normally cloudy (la garúa), but it never rains, as the marine front blocks all showers and thunderstorms. No moisture makes it over the Andes from the east, the mountains are too tall. According to a Chilean expert, Sagliani. "It is estimated that it has not rained in some places in the Atacama Desert in hundreds, even thousands of years." The surface is so like Mars that the Mars expedition test drove its robocars in the Atacama.

I was in Lima one winter, and there was no rain there either, but la garúa, the thick fog cap, blocked the sun for weeks on end. The only precipitation along the entire South American coast from Perú to Chile, is the condensation from la garúa. Not exactly rain falling in a rainshadow.

It's so weird to walk on bone dry dirt, there is no soil. It crunches under your feet like dried clay tailings or saltine crackers. I met people in Nazca who had never seen rain in living memory. But they told stories of deluges in Biblical proportions. And there are numerous ancient flash flood scars on the desert floor to support their stories. 

Sometimes rain happens in the Atacama Desert. It may not be related to global warming.

But the drought in southern Chile is certainly related to global warming. And this storm, destined for the Maipo vineyards of southern Chile, snuck past the desert goal keepers to weep in the driest of deserts and the world's largest open copper pits. Meanwhile, Villarica, a volcano in the south, which erupted on March 3rd, is rumbling again.

I gleaned my bits from several sources, but this webpage is the most enlightening.

See also
La Garúa
Atacama Civilizations
Chilean Miners

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