Definition
The condition in which a parcel of air contains the maximum amount of water vapor it can hold at a given temperature and pressure. At saturation, the relative humidity is 100 percent, and any further cooling or addition of moisture will cause water vapor to condense into visible moisture such as fog, cloud, or dew.
Plain English
The air is holding all the water vapor it can at its current temperature. Add any more moisture, or cool the air any further, and the extra water has to come out as tiny droplets you can see.
Context Anchor
Seen in fog, cloud, humidity, and temperature discussions, especially when explaining why fog forms near the ground.
Derivation
From the Latin saturare, meaning 'to fill' or 'to soak.' The same root gives us 'saturated' in everyday speech, as in a sponge that can't hold any more water. The aviation meaning keeps that core idea: the air is full and can't hold any more water vapor.
Why Pilots Care
Saturation is the point at which fog or clouds form, directly reducing visibility and triggering instrument flight requirements.
Grounding Statement
When temperature and dew point are equal, the air is saturated -- and that's when fog or cloud forms.
Intuition Check
Saturation does not mean rain is falling or that the air has turned into liquid. It means the air has reached its moisture limit for its current temperature and pressure.
Example Sentence 1
As the surface cooled overnight, the air reached saturation and radiation fog formed across the airfield.
Example Sentence 2
The forecaster noted saturation at the surface, warning pilots of likely low ceilings and reduced visibility.