Humidity

Humidity measures the water vapor present in air, expressed in forms such as relative humidity, absolute humidity, and humidity ratio.

Water vapor: Water in its gaseous state, dispersed as individual molecules in a gas. Humidity measures this gas when it is mixed with air.
Relative humidity: The ratio of water vapor pressure in air to saturation vapor pressure at the same temperature, usually expressed as a percentage. It is the most familiar humidity measure and changes when temperature changes.
Hygrometer: An instrument that measures humidity in air or another gas. Different hygrometers estimate humidity using changes in electrical, mechanical, or thermal properties.
Heat index: An apparent-temperature measure combining air temperature and humidity to estimate how hot conditions feel. High humidity slows sweat evaporation, increasing heat stress at a given air temperature.
Partial pressure: The pressure contributed by one component of a gas mixture. Water vapor pressure is one way to quantify vapor in air.
Absolute humidity: The mass of water vapor per unit volume of air. It measures vapor concentration directly rather than relative closeness to saturation.
Psychrometer: A humidity instrument that compares temperatures from a dry-bulb thermometer and a water-wetted bulb. Evaporative cooling of its wet bulb reveals the air's moisture level.
Cloud formation: The development of visible cloud droplets or ice crystals from water vapor in the atmosphere. Saturated humid air can condense on particles and form cloud droplets.
Saturation vapor pressure: The equilibrium vapor pressure of a substance above its liquid or solid at a given temperature. Relative humidity compares actual vapor pressure with this temperature-dependent limit.
Humidity ratio: The mass of water vapor per unit mass of dry air in a moist-air mixture. This mass-based measure remains useful when air is heated or cooled without gaining moisture.

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