How Do You Know Which Clouds to Target?

What is cloud-seeding? Is it artificial rain?
No, it’s natural rain that is precipitated by introducing certain elements into the cloud. Every raindrop is built around a nucleus called the cloud condensation nuclei (CCN). This particle forms the core around which there is a cloud deposit. We need these nuclei to form the raindrop. In nature, salt from the oceans, volcanic ash, even car exhaust yields the particles that function as CCNs. When we seed clouds, all we’re doing is introducing these nuclei by artificial means.

Indians have traditionally resorted to yagnas and havans for rain. Does this help?
Surprisingly, there could be a grain of science in this. Sitting on top of a mountain and starting a fire with various herbs and other materials thrown in, could, in theory, send particles into the clouds that become CCNs. The co-relation between smoke and rain was first observed during the US civil war. They found that it usually rained at the end of the day after a lot of cannon-firing. This, we now know, is because the dust and smoke created CCNs in the clouds and caused precipitation. Later, people began sending salt into the clouds by using kites and artillery shells.

How do you know which clouds to target?
Clouds are selected based on their water content and the thermal activity around them. Aircraft can then target the right kind of clouds. What helps tremendously here is weather radar. Advanced software has enabled us to study the cross sections of clouds to determine whether they are ready for seeding.
There are two types of clouds: Warm clouds, which are lower down and are above freezing point and cold clouds, which are below freezing point. Warm clouds are seeded using sodium chloride (common salt) and silver iodide is used for cold clouds.

With cloud-seeding via airplane, who leads the process, weather scientists or the pilot?
It is a combination of both. We absolutely have to look at the meteorological charts for the day to analyse the cloud cover in the region. There cannot be any rain if there are no clouds of the right type. The plane takes off only if the conditions are right, it reaches the base of the cloud and looks for an updraft (warm air going upwards). Then it flies upwards doing tight turns and releases the salt. If all goes well it starts raining within 25 to 30 minutes.

What are observable negative aspects of seeding clouds?
This is climate engineering. The reality is that we have already used up surface and ground water resources. Aerial water resources are the only ones yet to be tapped. A bad monsoon can severely impact an agricultural economy like ours. But there are no answers yet on the ill-effects. Scientists are still trying to study the impact of this process. In the US, there is a licensing policy and zones are demarcated between various states.

(As told to Cuckoo Paul)