Dew point

The dew point (also called dewpoint temperature) is the temperature at which an air parcel reaches saturation as it is cooled at constant pressure while retaining its water vapour. At saturation, the parcel's actual water-vapour pressure equals the saturation vapour pressure for that temperature. Continued cooling generally requires some vapour to condense or deposit, provided suitable surfaces or particles are available. If the dew point is below 0 °C, the corresponding saturation condition may be described using ice and the term frost point is sometimes used.

Dew point and relative humidity describe moisture in different ways. Relative humidity is a percentage comparing the water vapour present with the saturation amount possible at the current air temperature. Since warmer air has a higher saturation vapour pressure than cooler air, relative humidity can change substantially as temperature changes even if water-vapour content remains nearly the same. Dew point more directly tracks the moisture content of a near-surface air mass: for a fixed pressure, a higher dew point generally indicates more water vapour. Thus, a cool air mass can have very high relative humidity while containing less moisture than a warm air mass with a lower relative-humidity percentage. Comfort depends on several factors, but a higher dew point often makes warm weather feel more humid because it reduces the air's capacity to accept evaporated sweat.

When air is cooled to its dew point, saturation is reached. Cooling near the ground can lead to dew or fog, while rising air often cools as pressure decreases; if it reaches saturation, cloud droplets can form. The actual outcome depends on atmospheric conditions and the presence of condensation nuclei. Dew point is therefore useful to weather observers and forecasters, but it is not by itself a forecast of rain: cloud development and precipitation also depend on lifting, stability, cloud depth, and other processes. A thermometer and hygrometer, a psychrometer, or other humidity instruments can be used to measure or estimate moisture variables from which dew point is determined.

Meteorological observations commonly report air temperature and dew point together. Their difference, called the dew-point depression, indicates how close the air is to saturation at the observation level: a small difference means conditions there are nearer saturation, though it does not alone establish that fog or precipitation will occur. Forecasters use dew point and its spatial pattern to assess low-level moisture, potential condensation, and the character of an incoming air mass. The American Meteorological Society notes that dew point remains unchanged during isobaric heating or cooling if vapour is neither added nor removed, but can change as air rises or sinks and its pressure changes.