10. For any air parcel, there is a specific temperature at which the air will become saturated with water vapor. This is the dew point temperature.

When the ambient (surrounding) temperature is higher than the dew point temperature, a parcel of air is said to be undersaturated, and evaporation outpaces condensation. When the temperature has dropped to the dew point, the rates of evaporation and condensation are equal and the air is said to be saturated or to have reached its capacity with respect to water vapor. At temperatures below the dew point, the air is supersaturated and condensation outpaces evaporation. The carrying capacity of air with respect to water vapor is dependent upon its temperature. The higher the temperature, the greater the carrying capacity; that is the more water vapor it can hold.

When surface air is warm, more water at ground level will evaporate. if the air is warm, it can hold a great deal of water vapor; when cold, air can hold only a comparatively small amount of water vapor. Therefore, cloud formation (Figure 1) and precipitation (rain, snow, hail, etc.) occur only when air cools sufficiently to make some of the vapor condense back to its liquid form (water) or be deposited in its solid form (ice) and fall to the ground. This is basically the same process that causes some freezers to require  periodic defrosting. Warm air holding much vapor rushes into the freezer when the door is opened. Inside the freezer the air is cooled considerably, and the vapor crystallizes to form frost on the freezer walls.

Figure 1. Expansional cooling of warm, moist,  convecting air causes condensation to occur.  If sufficient condensation takes place, clouds  will form.