Artificial rainfall is caused by ionized particles moving up from the ground and attracting water vapor from the ambient air to form clouds
Large ionizers produce negatively charged particles or electrons. These particles naturally attract dust particles present in the ambient air in desert and dry arid regions.
Those particles then move up and away from the ionizers as a result of the hot ground air moving and forming convection currents as a result.
As the particles reach the height around which the clouds usually form, they stabilize and the negative charge of the particles begins to attract the vaporized water molecules around resulting in the formation of mini clouds. The particles act as nuclei around which all the moisture can come together, the process is very similar to how crystals are formed. Gradually, the size of those mini clouds grows as more particles move up and attract more of the humidity
For this process to work, the ambient humidity should be at least 30%
These clouds being to rain, as the temperature starts to cool down, mostly during night times in the desert. The resulting rain will be combined with thunder and lightning, as the charged particles act as giant electrodes in the sky across which the lightning discharges
This process of artificial rainfall creation is more interesting and relevant for dry regions or deserts. For regions with more cloud formation, but little rain, the artificial rainfall approach to cause rains is to seed the clouds with chemicals (sprayed with small planes flying through the clouds) that will act as nuclei for the water drops to congeal around until they become heavy enough to start falling down in a beautiful and all artificial rainfall