Ar­ti­fi­cial ice crys­tals force wa­ter out of the sky

China is car­ry­ing out the great­est weather ex­per­i­ment ever. With sil­ver io­dide pow­der, me­te­o­rol­o­gists aim to make ma­jor quan­ti­ties of pre­cip­i­ta­tion fall on the Ti­betan Plateau, so Asia’s largest wa­ter reser­voir will not run dry.

Science Illustrated - - ARTIFICIAL RAIN -

Ev­ery­body is talk­ing about the weather, but no­body can do any­thing about it – ex­cept for China. The world’s most pop­u­lous na­tion is car­ry­ing out his­tory’s most ex­ten­sive me­te­o­rol­ogy ex­per­i­ment to en­sure vi­tal pre­cip­i­ta­tion for one of the world’s dri­est re­gions.

The weather mak­ers aim to force the clouds above the Ti­betan Plateau in South-West­ern China to give off huge quan­ti­ties of rain and snow to the 1.6 mil­lion km2 re­gion.

The large-scale project is known as Tianhe – “Sky River” – and is still in its ini­tial stage. On steep moun­tain slopes in Ti­bet, sci­en­tists from the government-owned China Aero­space Sci­ence and Tech­nol­ogy Cor­po­ra­tion and a team from the Ts­inghua Univer­sity in Bei­jng have in­stalled 500 rain­mak­ing ma­chines, which are com­bus­tion cham­bers in­clud­ing a sil­ver io­dide so­lu­tion. Sil­ver io­dide has the same struc­ture as ice crys­tals and func­tions as a type of dou­ble of real ice. When heated, the warm air car­ries sil­ver io­dide par­ti­cles to the clouds. In the clouds, the par­ti­cles make the ex­ist­ing wa­ter vapour con­dense into wa­ter drops, which grow big­ger, only fi­nally to fall as rain or snow.

In the years to come, tens of thou­sands more com­bus­tion cham­bers will be in­stalled on the Ti­betan Plateau to sup­ply the clouds with sil­ver io­dide par­ti­cles. The method is known as cloud seed­ing, and it is a rel­a­tively sim­ple and cheap way of ma­nip­u­lat­ing the weather. Each com­bus­tion cham­ber costs about $10,000, and if ev­ery­thing turns out the way sci­en­tists ex­pect it to, the in­vest­ment will in­deed be worth­while. Ac­cord­ing to cal­cu­la­tions, the an­nual pre­cip­i­ta­tion in the drought-rid­den plateau will in­crease by up to 10 bil­lion m3 an­nu­ally.

Satel­lites mon­i­tor the mon­soon

The Ti­betan Plateau is lo­cated at an alti­tude of 4,000-5,000 m above sea level. It is known as the wa­ter tower of Asia, and justly so. Ten of the con­ti­nent's largest rivers orig­i­nate on the huge plateau, sup­ply­ing about 1.3 bil­lion peo­ple with fresh wa­ter. Global warm­ing made tem­per­a­tures in the plateau rise by an av­er­age of 1.9 de­grees in 1961-2012, and in sev­eral places, the pre­cip­i­ta­tion is now less than 10 cm an­nu­ally, cor­re­spond­ing to the quan­tity of pre­cip­i­ta­tion fall­ing on the Saraha Desert in one year. Ac­cord­ing to cli­mate re­searchers,

tem­per­a­tures in the Ti­betan Plateau will have risen by five de­grees in 2100, leav­ing mil­lions of peo­ple with­out drink­ing wa­ter. But al­ready now, China feels the con­se­quences of the lack­ing pre­cip­i­ta­tion.

In 2011, 40 % less rain than the usual level fell near the Yangtze River. The lack of pre­cip­i­ta­tion made its wa­ter level fall, de­stroy­ing much of the rice crop, shut­ting down sev­eral wa­ter­works, and halt­ing the sup­ply of drink­ing wa­ter for mil­lions of peo­ple.

The peo­ple be­hind the Heaven River project aim to avoid sim­i­lar dis­as­ters in the fu­ture. By means of 30 dif­fer­ent weather satel­lites, me­te­o­rol­o­gists will mon­i­tor the mon­soon ac­tiv­ity above the In­dian Ocean. When the weather sys­tem hits the Ti­betan Plateau, the com­bus­tion cham­bers are ac­ti­vated, so the wind car­ries the sil­ver io­dide par­ti­cles to the clouds to cause rain. And if the com­bus­tion cham­bers are not enough, the clouds could al­ways get ex­tra sil­ver io­dide from the fleet of planes, drones, and rock­ets that also form part of the Heaven River project.

Works bet­ter than rain dance

China is not by far the only na­tion act­ing as a rain­maker th­ese days. Since Amer­i­can chemist Vin­cent Schae­fer in­vented cloud seed­ing in 1946, the in­ter­est in weather ma­nip­u­la­tion has risen steadily. Ac­cord­ing to a re­port pub­lished by the World Me­te­o­ro­log­i­cal Or­ga­ni­za­tion, 56 coun­tries are car­ry­ing out cloud seed­ing projects.

Apart from China, which is the world’s lead­ing cloud seed­ing na­tion with some 55 bil­lion t of ar­ti­fi­cially-pro­duced rain an­nu­ally, par­tic­u­larly the United Arab Emi­rates are count­ing on cloud seed­ing. The oil-rich na­tion not only ma­nip­u­lates the clouds above its own deserts – it is also hired by coun­tries such as Aus­tralia and the US to in­crease pre­cip­i­ta­tion above some of the coun­tries’ most thirsty agri­cul­tural ar­eas.

It is not only sil­ver io­dide that is used to seed the clouds. Na­tions such as Thai­land and the US also use dry ice or salt, such as potas­sium

chlo­ride or sodium chlo­ride that also make wa­ter vapour con­dense in the clouds and col­lect around salt. As a gen­eral rule, sil­ver io­dide and dry ice are used in high-ly­ing, cold clouds, whereas salt is used in warm, low-ly­ing clouds.

Although the chances of more rain are very much wel­comed by mil­lions of peo­ple, ev­ery­body is not equally thrilled.

Iran’s for­mer pres­i­dent, Mah­moud Ah­madine­jad, on sev­eral oc­ca­sions ac­cused Europe of de­lib­er­ately us­ing cloud seed­ing to steal rain from the thirsty Ira­nian peo­ple.

In China, rain­mak­ers have also been much de­bated. In 2004, me­te­o­rol­o­gists seeded the clouds out­side the city of Pingding­shan in the drought-rid­den He­nan prov­ince. Half an hour later, 100 mm of rain fell on the thirsty in­hab­i­tants. But when the clouds reached the neigh­bour­ing city of Zhoukou, only 27 mm of rain re­mained in the clouds, mak­ing the ci­ti­zens ac­cuse the me­te­o­rol­o­gists in Pingding­shan of hav­ing stolen their rain.

Ac­cord­ing to scep­tics, cloud seed­ing is no bet­ter than a rain dance. Any weather sit­u­a­tion is unique, and it is dif­fi­cult to tell if the rain falls due to nat­u­ral pro­cesses, or be­cause the clouds have been seeded. But in early 2018, US sci­en­tists man­aged to prove the ef­fect of cloud seed­ing by means of radar mon­i­tor­ing. The data showed that the clouds which had been seeded with sil­ver io­dide pro­duced 1001,000 times more ice crys­tals than the clouds that had not been ma­nip­u­lated.

Pump­ing the clouds full of chem­i­cals does not im­me­di­ately sound very eco-friendly, but ac­cord­ing to sci­en­tists, cloud seed­ing has no neg­a­tive ef­fect on the en­vi­ron­ment. They have car­ried out more than 30 stud­ies con­cern­ing the ef­fect of cloud seed­ing on the en­vi­ron­ment, and no con­tam­i­na­tion has been spot­ted.

China’s wa­ter cri­sis evap­o­rates

It re­mains un­clear, when China will se­ri­ously be­gin the ma­jor weather ex­per­i­ment, but to many of the 1.3 bil­lion peo­ple wait­ing, the rain can­not come fast enough.

In China, 400 cities lack wa­ter, and ac­cord­ing to a Green­peace re­port, agri­cul­ture will shrink 23% by 2050 as a con­se­quence.

Ac­cord­ing to Chi­nese me­te­o­rol­o­gists, the thou­sands of com­bus­tion cham­bers in the Ti­betan Plateau will soon end China's wor­ries. It seems un­likely that such a huge prob­lem can be solved so eas­ily, and there will no doubt be many twists and turns along the way. But, just like wa­ter it­self, that's life.

Me­te­o­rol­o­gists fre­quently seed the clouds of North Dakota, USA, with sil­ver io­dide and dry ice from planes.

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