Scientists have been trying to develop reliable methods of cloud seeding to make more rain fall from clouds. In the technique investigated the most, aircraft drop artificial freezing nuclei into clouds. The usual seeding material has been silver iodide, whose crystals have a structure much like that of ice. The earth as a whole receives abundant rainfall. But rainfall is unevenly distributed over the earth's surface.
Distribution is especially uneven over the continents because of mountain ranges.
Heavy rain often drenches slopes where the air rises, leaving dry the slopes where the air descends. The convergence causes the air to rise, producing clouds and rain. Certain regions alternate between rainy and dry seasons due to shifting winds. In regions near the tropics, winds known as monsoons blow in one direction in the winter and in the opposite direction in the summer. Monsoon winds bring extremely heavy rain to southern Asia in the summer.