Dew point

The temperature at which air becomes saturated with water vapor when cooled at constant pressure. It indicates the air’s actual water-vapor content.

Saturation vapor pressure: The equilibrium vapor pressure of a substance above its liquid or solid phase at a given temperature. Its temperature dependence determines when water vapor saturates cooling air.
Fog: A cloud of suspended water droplets or ice crystals at Earth's surface that reduces visibility. Fog forms when near-surface air cools to saturation, often reaching its dew point.
Water vapor: Water in its gaseous phase, present in the atmosphere in amounts that vary across places and time. Dew point reflects the amount of water vapor in an air parcel.
Wet-bulb temperature: The lowest temperature attainable by evaporative cooling of a surface exposed to air. It is generally above dew point and measures evaporative cooling rather than saturation by cooling.
Condensation: The phase transition in which a gas becomes a liquid. Water begins condensing when air cools to its dew point.
Cloud condensation nuclei: Tiny atmospheric particles on which water vapor condenses to form cloud droplets. They help initiate droplets after air reaches saturation near its dew point.
Atmospheric pressure: The force per unit area exerted by the weight of the atmosphere. The dew-point definition specifies cooling at constant pressure.
Relative humidity: The ratio of air’s current water-vapor pressure to its saturation vapor pressure at the same temperature. Relative humidity changes with temperature, while dew point more directly reflects moisture amount.
Clausius–Clapeyron relation: An equation describing how equilibrium pressure between phases changes with temperature. It quantifies how rapidly saturation vapor pressure falls as air cools.
Heat index: An apparent-temperature measure combining air temperature and humidity to estimate heat stress. Dew point provides a practical indicator of the moisture that makes heat feel oppressive.

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