The idea of cloud seeding has once again caught public imagination as a technological solution that could bring relief from Delhi’s air crisis.

The concept appears simple enough: disperse tiny particles such as silver iodide or sodium chloride into the atmosphere so that they act as condensation nuclei, encouraging water droplets to form, merge, and fall as rain.

Yet, the science of rainfall is never that simple. For cloud seeding to succeed, several delicate conditions must align — adequate moisture, the right type of cloud, a suitable temperature range, gentle updrafts, and just enough time for condensation to mature into rainfall. Wind speed, the height and density of the cloud, the concentration of existing aerosols, even the electrical charge within the cloud, all influence whether raindrops will grow or evaporate mid-air. Almost all of these factors are beyond human control. We cannot create clouds or dictate wind. We can only wait for a window of opportunity and intervene modestly.

That is why the success rate of cloud seeding seldom crosses twenty per cent if we remove rainfall........

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