Precipitation is the process by which water from the atmosphere falls back on earth in liquid or frozen form. It is one of the four stages of the water cycle, which also includes evaporation, condensation, and infiltration or runoff. Precipitation can occur in the form of rain, sleet, snow, or hail, and it is determined by the temperature of the cloud and the temperature on the earths surface. Dust or smoke particles in the atmosphere form the condensation nuclei and provide a surface for water vapor to condense upon, which helps water droplets gather together and become large enough to fall to the Earth. Precipitation is essential for life on Earth, as it replenishes freshwater resources and supports plant growth.