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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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