What often occurs when clouds form upon reaching 100% relative humidity. If condensation nuclei are present, liquid or solid particles of water will form. When this material becomes heavy enough to fall towards the ground, it is known as the common phenomenon snow, rain, sleet, or hail.
[Nature; v.372; pages 250-252; 1994.] [Environmental Science and Technology; v.28; pages 1459-1466; 1994.]
Source: Atmospheric Chemistry Glossary