Dew Point
Definition and meaning of Dew Point in chemistry.
Dew point is the temperature to which a parcel of air must be cooled, at constant pressure and constant water vapor content, for the air to become saturated with water vapor, causing condensation to begin.
In more detail
As air cools, the maximum amount of water vapor it can hold (the saturation vapor pressure) decreases. The dew point is reached when the actual partial pressure of water vapor in the air equals this saturation vapor pressure, giving a relative humidity of 100%. Cooling below the dew point forces excess water vapor to condense into liquid droplets (dew, fog, or cloud) or, below 0°C, to deposit as frost. A higher dew point indicates more water vapor in the air and is a more direct measure of humidity than relative humidity, since relative humidity also changes with air temperature alone.
Key facts
| Field | Physical Chemistry |
|---|---|
| Condition | Relative humidity = 100% at the dew point |
| Depends on | Partial pressure (concentration) of water vapor in the air |
| Below 0 °C equivalent | Frost point (deposition instead of condensation) |
On a clear night, if the air temperature is 25°C and the dew point is 18°C, the air must cool to 18°C before moisture condenses; once the surface temperature of grass or a car windshield drops to 18°C, dew forms on it.
Frequently asked questions
How does dew point differ from relative humidity?
Dew point is an absolute measure of the water vapor content of air (a temperature value), whereas relative humidity is a ratio that depends on both the water vapor content and the current air temperature. Two air masses can have the same relative humidity but very different dew points if their temperatures differ.
Why does condensation form when air reaches its dew point?
At the dew point, the air's water vapor partial pressure equals the saturation vapor pressure for that temperature, so the air can hold no additional vapor; any further cooling shifts the equilibrium and excess water vapor condenses into liquid droplets.