Currently there are not many ways in which humanity can control rainfall in specific areas, but there are a few methods that mimic the natural production of rain. Of course there are specifications to such techniques that make the process even more difficult. First, there needs to be abundant quantities of clouds in the targeted areas, a factor which would do nothing to satisfy a water-deprived area beneath clear skies. Second, the temperature and updraft effects have to be of the right consistency, or else there may be an undesired formation of snow or hail storms rather than rain.
Before getting to the methods for invoking rainfall, it is necessary to describe the factors that produce rain. Most people are aware that clouds are made up of water moisture that hangs in gathered quantities in the sky. Rather than being liquid water, clouds actually consist of very small ice crystals that are held in the cooler upper atmosphere of the sky via thermal updrafts from the earth below. Depending on the strength and consistencies of the clouds, they will either be higher or lower. Apart from the ice can be scatterings of dust or other small particles which will eventually form the core of a potential raindrop. The common name for these cores is nuclei.
With the stronger thermal updrafts from the warm lands below, larger pieces of ice are allowed to form using the nuclei as a base and adding to it other pieces of ice or super-cooled droplets of water. Once the larger pieces of ice become too heavy to remain supported on the updrafts, gravity wins over and the ice falls. If the temperature between the cloud and ground is warm this ice will melt and become rain, otherwise it will be hail. In order to control rainfall, humans duplicate this natural process.
Although several methods for initiating rain may exist, the most common form is known as Cloud Seeding. This process is further divided into two sub-classes, static and dynamic methods. In the static process colder clouds are provided with nuclei in order to stimulate the formation of rain. Usually dropped from a plane, the common agent silver iodide is released into the cloud formation. As the act of falling usually distributes the liquid into fine quantities, the lower freezing point quickly creates the perfect basis for rain formation. While in the dynamic method, the formation process is accelerated by introducing dust or other materials as nuclei, accompanied with cooling agents like liquid nitrogen or dry ice. Since the cooling process speeds up the formation of rain, it is a faster process.
While these methods are the best at our current disposal, they are not the most reliable. Also, the very nature of cloud seeding is argued upon, with opponents commonly stating that by forcing rain in one locale, it robs it from another – potentially more dire-in-need – area. As for other potential technologies that help control rain, none of the other proposed solutions contain as efficient or cost effective means.