How to make it rain
Humans can influence the weather ― to a degree. Today, cloud seeding, or artifical rain, is mostly used to bring water to drought-ravaged regions. But it's also been misused in the past.
Clouds form when air containing water vapor rises into the atmosphere, cools and forms icy particles. Once enough of those particles clump together, a cloud forms. Inside the cloud, the icy particles combine.
When the combined droplets have grown large and heavy enough, they fall to the ground as some form of precipitation: Rain, snow or hail, depending on the temperature and other weather conditions.
With cloud seeding, small particles of silver iodide, a salt with a crystalline structure similar to that of ice, are added to clouds. This process can be performed either from a plane or drone, or particles can be shot up from the ground.
The method allows the water vapor inside clouds to be "tricked" into forming droplets around the silver iodide particles, Jose Miguel Vinas, a meteorologist with Meteored, a Spanish company that runs weather websites in several countries, told DW.
Once the droplets become heavy enough ― a process that is accelerated by the addition of the silver iodide ― they drop from the clouds as precipitation.
The way the process works explains why Beijing is currently struggling to cloud seed: There is a need for at least some clouds to already be in the parts of the sky where you want to induce rain, and some of the regions in China that need water most desperately don't have enough cloud cover for the method to work. Humans still cannot create rain clouds out of thin air.