How to make rain. Yes, it is really possible

In ancient times, people went to shamans to make it rain, and more than half a century ago, they invented a technology for that. But is it really any more efficient than shamans? In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about rain making and how exactly to do it.

Seeds for clouds

To remind you, to turn into water or snow, water vapor in the clouds needs more than just low temperature. Vapor particles need a base to 'freeze' to and condense, that is, to switch to a liquid or solid state.

A base like this can be formed by dust, soot, pollen, and other small particles the wind spreads through the atmosphere. Colliding with them, vapor condenses, and later bonds with other similarly condensed particles near them. As a result, the ice crystal grows and, if it becomes heavy enough, it falls to the ground as rain or other precipitation.

Turns out, if you artificially spread small particles in the clouds and speed up condensation, you can make rain or snow? At least, that's what scientists thought in the early XXth century, and by the middle of the century they began experimenting with real weather.

Fitting 'seeds' — condensation core — turned out to be aerosols from dry ice and various salts, including ordinary table salt and silver iodide, and cement. Raising the concentration of these substances in the atmosphere to the scale necessary for seeding was deemed safe.

In 70 years, seeding and cloud dispersal principles haven't changed: planes, surface cannons or rockets spread condensation cores in the clouds, to make sure it doesn't rain or snow where it's nop supposed to.

Areas of use are enormous: from daught relief to saving crops from hail to lowering precipitation in cities before holidays and events like the Olympics, like in Beijing in 2008.

Once, cloud seeding was even used in a war: in a secret operation in late 1960's, the military tried to extend monsoon season in Vietnam to spoil the roads and lower the North Vietnam military's efficiency.

And even though that time the organizers of the operation reported success, scientific data on cloud seeding isn't so unambiguous.

Technologies vs. shamanism

Even though cloud seeding is widely used around the world, and the theory in its basis seems simple and solid at first glance, scientists have failed to convincingly prove that cloud seeding actually has a strong effect on precipitation.

Let's take shamanism: even if after a ritual it suddenly rains and everyone's happy, how do we know if it would rain without the ritual or not? Same with cloud seeding: meteorology can't say how much moisture one or another cloud will give out, the variables are to unpredictable. And that means it's hard to test if it worked or not — even if it did.

Most studies of the technology over the past 70 years showed either insignificant or nonexistent results. Scientists' work is also curbed by the fact that not all clouds are fit for seeding: for instance, you need temperatures cold enough, and clouds near atmospheric fronts won't work for cloud seeding.

For now, scientists were the luckiest with seeding in mountainous regions in the winter. The Snow mountains experiment in Australia, for instance, showed snow precipitation grew up to 14% after seeding, and in the 6-year experiment (LINK) in the mountains in Wyoming where seeded clouds were compared with un-seeded clouds, showed less impressive results — just 1,5% more snow than usual.

More large scale research is being prepared, but based on existing statistics, for now, we can say that even if seeding can get a bit more snow and rain out of a cloud, it depends on the area and other specific factors.

Text: Oleg Ovechkin, a journalist and a traveller

Cover photo: Alberto Casetta / Unsplash