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How is artificial rain made?

What's common to boiling water, artificial rain and the Mentos-Diet Coke reaction? It's nucleation! Let's see what that means.

So what is nucleation?

How does rain happen in nature? As wind passes over the sea, it collects evaporating water as very, very tiny drops. As the wind rises high in the air, temperatures become very cold, causing some of these drops to merge. This bigger drop attracts more drops of water, and so on and on. This process is called nucleation.

When the water drops become about 0.1 mm thick, they cannot float in the air anymore. They then fall to the ground as rain.

Artificial rain

Sometimes it does not get cold enough in the cloud for nucleation to occur. But if we want the cloud to rain, we have to create places where nucleation can occur. This is done by a method called cloud seeding.

Tiny particles of silver iodide are sprayed on a cloud from an aeroplane. Sometimes, solid carbon dioxide (called dry ice) is used, as it is cheaper. The particles attract water drops from the cloud. When they form a drop that is large enough, it starts raining. You can read more about cloud seeding here.

Environmental effects

Artificial rain has a few unintended consequences. The use of silver iodide means that there is a lot of silver that also rains over the landscape. If cloud seeding is done for many years in a place, this silver begins to accumulate in plant and animal bodies. Too much silver in the body leads to a condition called argyria. In this, the skin and eyes get a bluish hue.

Silver iodide is a hazardous chemical, and can cause anaemia, weakness and loss of weight. Hence the worker must be completely protected when handling silver iodide.

More on Nucleation

You might have heard of the famous Diet Coke-Mentos reaction. Pop a Mentos candy into a Diet Coke bottle, and watch it fizz! Here's a video:

(Video source: http://www.youtube.com/watch?v=LjbJELjLgZg)

The answer is nucleation, just as in clouds. If you look at a Mentos candy under a microscope, you will see that it has a rough surface. This surface creates nucleation sites for the carbon dioxide dissolved in the Diet Coke. Bubbles begin to form on the surface of the candy. When they become too big, they escape from the bottle. Millions of bubbles like that, and you get to see a big fizz.

The same thing happens when you boil water. As the water becomes steam, tiny bubbles begin to form. Tiny dents in the vessel used act as nucleation sites, where the bubbles become bigger till they escape.

If you boil water in a scratch-free microwave bowl, it tends to heat up over 100°C without boiling. But it will boil over immediately if you shook it or put a spoon in it (Don't ever put your finger). That's because you are introducing a nucleation site for the steam bubbles to start growing. So if you boil water in a microwave, be sure to use mittens or tongs.

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