The role of silver iodide in artificial rainfall is called cold cloud catalysis in meteorology. As long as silver iodide is heated, it forms very fine silver iodide particles (only 1% to 1/1000 of the diameter of a hair) in the air. 1g silver iodide can form trillions of particles. These particles move with the airstream into the cloud, producing trillions to trillions of ice crystals in cold clouds. Therefore, using silver iodide to catalyze rainfall does not need aircraft, the equipment is simple, the dosage is very small, the cost is low, and can be popularized in a large area.
According to this passage, the reason why airplanes are not needed for silver iodide to catalyze rainfall is that silver iodide particles will move into the cloud with the airflow.