Why does precipitation occur when warm air rises?

Warm air masses naturally rise above cold air, due to density differences. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, water vapor condenses and forms extensive cloud coverage. The first clouds to form along the sloping surface of the cold air are high cirrus, which thicken to cirrostratus and altostratus. Once the clouds have thickened to 2500 m from the earth's surface, rain can begin to fall from the heavy nimbostratus cloud.