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  • It was 8:00 p.m. local time, when the opening ceremony for the 2008 Summer Olympics was

  • kicking off, and the sky was totally clear of clouds. The Bird's Nest Stadium does

  • not have a roof, and possible rainfall would have been an imperfection during this perfectly

  • orchestrated $100 million USD show. But the clear skies, were no coincidence. Starting

  • From 4 hours before the show began until late at night, any clouds near the city limits

  • of Beijing were shot down using rockets and chemicals.

  • But how is that possible?

  • Clouds form from the evaporation of water, which then rises, cools and condenses. But

  • in order to condense the water droplets need a solid to which they can adhere to. These

  • can be tiny particles such as dust floating in the air. If more and more water accumulates

  • on this nuclei and the drop grows, it becomes heavier until eventually it will fall to earth

  • as rain. The process of helping that process out by

  • using chemicals is known as cloud seeding. In order to achieve this, additional particles

  • or nuclei where the water molecules can attach to are added. This can be achieved using airplanes

  • equipped with pyrotechnic flares that hold chemicals like salts, dry ice, or silver iodide,

  • which is commonly used. When the plane flies over a cloud layer, the chemicals can be scattered

  • and the swirling winds behind the plane help distribute the particles.

  • Clouds are particularly suitable for this if they contain a high amount of supercooled

  • water. This means it has a temperature that is below the freezing point of water, but

  • it is still in liquid form. That is because it is very difficult for the water to transition

  • from liquid to solid if there is no nuclei from which the crystal structure can start

  • to grow. By adding particles, such as silver iodide, the nucleation process can begin.

  • In fact, this can even be achieved simply by flying an airplane through a cloud layer,

  • without spraying any chemicals. The resulting shock itself can help the molecules to line

  • up and trigger the freezing process. his is the effect that can also be observed in those

  • videos that show water bottles suddenly freezing due to a shock.

  • This satellite image shows a similar effect. The cloud layer appears perforated with so-called

  • canal clouds and hole-punch clouds. These are the result of aircrafts that flew through

  • the cloud and triggered small bursts of rain or snow that leave behind gaps in the cloud

  • cover. At the 2008 Olympic Games, however, no planes

  • were used. Instead a total of 1104 rockets containing chemicals were fired into the clouds.

  • But among experts there is an ongoing debate regarding the effectiveness of cloud seeding.

  • It very difficult to conduct studies on this as the crucial question is whether it would

  • have rained the same way or not without any intervention. The difficulty of making high

  • accuracy predictions regarding weather makes it really hard to find statistically solid

  • information about how effective cloud seeding is.

  • But nevertheless there are a plenty of cloud seeding projects all around the globe. Especially

  • in dry regions that are plagued by lack of water, nations hope that this technology allows

  • them to force clouds that would otherwise move on to condense and rain. The United Arab

  • Emirates for example, a nation with desert climate, operates such a program on the national

  • level and is financially supporting further research on rain enhancement technologies.

  • Cloud seeding is also used in an attempt to mitigate extreme weather events, like severe

  • droughts. For example, Thailand has been operating a government-run rainmaking program since

  • 1955. It was introduced in order to secure crops as most agricultural Land in Thailand

  • relies on rainfall. At the same time, cloud seeding is also used to minimise extreme floods.

  • This is achieved by targeting clouds early, when such floods are predicted. As happened

  • in early 2020 in Indonesia after a storm flooded large parts of Jakarta.

  • But there was also a time where cloud seeding was used for a very different goal. Using

  • the weather as an advantage during war. This is what happened during the Vietnam War, as

  • part of a secret US tactic: Operation Popeye. The goal was to extend the monsoon season

  • and therefore create strong floods that damage roads and cause landslides. The US especially

  • targeted the Ho Chi Minh Trail, which was the primary supply route used by North Vietnam

  • in order to infiltrate troops, weapons and supplies into South Vietnam.

  • Using cloud seeding as a weapon of war, causing enormous damage to civilians, raised a number

  • of moral questions. And in 1977, an international agreement was signed by many nations worldwide

  • that prohibits the military use of such means.

  • Although cloud seeding is already used widely, scientific discussions regarding the effectiveness

  • remain. And there is also a discourse on possible health and environmental risks. Then there

  • are political questions that need to be answered, such as whether a country holds rights to

  • the clouds that move over its territory. It's are theoretical questions, but given the urgency

  • of water supply in many regions and the water conflicts already ongoing due to rivers that

  • flow through several nations, this could lead to disputes in the future.

  • As climate change leads to more extreme weather events, it will be interesting to see whether

  • there will be more research and investment into this area.

  • If you want to learn more about strategies to combat the critical challenges resulting

  • from climate change, I highly recommend "Climate Impact Asia", a four part documentary series

  • examining how some of the most vulnerable regions to climate change approach these challenges.

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How Cloud Seeding Is Used to Make It Rain

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    joey joey に公開 2021 年 06 月 15 日
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