The temperature at which air becomes saturated and water vapor condenses is called the dew point.
At the dew point, air has reached 100% relative humidity for its current pressure and moisture content. Cooling air reduces the amount of water vapor it can hold; once the temperature reaches this threshold, excess vapor changes into liquid droplets. Dew on grass, fog near the ground, and many clouds are familiar results.
Dew point is not the same as relative humidity. Relative humidity is a percentage comparing the air’s vapor content with its maximum capacity at a given temperature, while dew point is an actual temperature. A high dew point indicates moist air and often feels muggy; a low dew point indicates drier air.
If temperatures fall below freezing, deposition can produce frost rather than liquid dew, so meteorologists may refer to the frost point. John Aitken’s 1887 experiments helped establish that dew forms from moisture in the air rather than falling from the sky.