This document discusses the four main mechanisms that cause precipitation: 1) Convectional lifting caused by surface heating, 2) Frontal wedging when contrasting air masses collide, 3) Orographic lifting as air is forced upwards by mountains, and 4) Convergence when winds from different directions collide and force air to rise. Precipitation occurs when the lifting mechanisms cause air to cool and reach its dew point, condensing moisture into clouds and rain or snow.