We have learned that water is available on the Earth in the following three forms: vapor; liquid; and solid. The process of water moving from one of these forms to another is called a phase or state change. In the atmosphere, three processes act to create water droplets or ice crystals. These three processes are:
Condensation - water moving from a vapor to a liquid state.
Freezing - water moving from a liquid to a solid state.
Deposition - water moving from a vapor to a solid state.
For a phase change to occur heat energy must be added to or removed from water molecules. The formation of water droplets and ice crystals takes place when the water in the atmosphere is cooled. As air containing water vapor cools, the relative humidity of the air parcel increases until the dew or frost point is reached. At dew point (relative humidity = 100%) water begins to condense into droplets. If 100% relative humidity is reached below 0° Celsius deposition occurs and ice crystals form.
Formation of water droplets and ice crystals also requires a surface for condensation, freezing, or deposition. In the atmosphere, these surfaces are microscopic particles of dust, smoke, and salt commonly called condensation nuclei. Deposition nuclei, six sided particles, are needed for the formation of ice crystals.
If there is a deficiency of nuclei, super-saturation can result and condensation, freezing, or deposition can only occur with a relative humidity that is greater than 100%