Moisture in the clouds condenses and collects together to form rain. And moisture needs a central molecule to wrap itself around. These particles are called cloud condensation nuclei, which are normally sea salt, clay silicate particles and smoke particles. So, if you want rain, you either wait for it to happen naturally. Or create it artificially by putting these particles in the clouds.

Why & How it rains

The warm air that holds these particles rises. As this warm air rises higher in the atmosphere, it cools. The water vapour condenses around the particles into droplets of water and small crystals of ice. These particles keep collecting and growing in size until it reaches a saturation point. Like a balloon filled to its fullest with air. A cloud burst then occurs and the water droplets begin to fall on earth. As they fall, the droplets collide with smaller ones to form bigger droplets in a process called coalescence. And that's how the water droplets reach us as rain.

Silver iodide was used for cloud seeding in the Beijing Olympics

Making it rain

Cloud seeding is a technique that helps build clouds and produce rain. It works well when there are clouds that consist of liquid water droplets at a temperature that is below the freezing point of water. As nuclei particles are introduced artificially, ice crystals form around them. They then become heavier and fall rapidly to the earth. As they fall, they melt and become rain.

So, when was the first cloud seeding done?

The first cloud seeding was conducted in 1946 when two chemists at General Electric Research Laboratories in New York did an interesting experiment. Irving Langmuir and Vincent J Schaefer discovered that dry ice pellets could help form big ice crystals in a cloud. The cloud in their experiment consisted of water droplets in a deep-freeze box. As the pellets fell through this super cooled layer of cloud, water droplets present in the cloud froze. The ice crystals formed grew in size till they became so large that they fell to the bottom of the freezer. And that was the first ever cloud seeding.

What's used in cloud seeding?

Apart from dry ice pellets, Silver Iodide is an alternative substance used for cloud seeding. It is an effective particle because the crystal of Silver Iodide is similar to that of ice. This means that ice can easily gather on these particles.

How is it done?

The particles can be dispersed in clouds either by spraying from aircrafts or by using ground based rocket launchers.

Can cloud seeding happen on its own?

Yes! Cloud seeding occurs naturally when forests burn. This is because the smoke that is created is full of particles that help in naturally seeding clouds. This is why often forest fires are put out by rain.

When was it done recently?

During the Beijing Olympics in 2009. Silver Iodide particles were seeded into clouds, creating rain in places around Beijing to ensure the opening ceremony itself would not be affected by rainfall. These particles helped in increasing the number of hailstones, while reducing their size at the same time. This means rain is less likely to fall. This technique is used sometime to reduce fog at airports.