Psychrometrics
Psychrometrics studies the thermodynamic properties of moist air and the relationships among temperature, humidity, pressure, and energy.
Psychrometric chart: A graphical plot of moist-air properties, commonly used at a fixed atmospheric pressure. Its intersecting property lines make psychrometric relationships usable for tracing air-conditioning processes.
Water vapor: The gaseous phase of water, present in air in amounts that vary with conditions. Its partial pressure and mass determine the air's moisture state.
Heating, ventilation, and air conditioning: The engineering of indoor environmental control through heating, ventilation, and air-conditioning systems. HVAC uses moist-air calculations to set comfort conditions and size equipment.
Specific humidity: The mass of water vapor divided by the total mass of a moist-air mixture. It resembles humidity ratio but uses total moist-air mass rather than dry-air mass.
Humidity ratio: The mass of water vapor per unit mass of dry air in a moist-air mixture. It tracks the water content that remains conserved during sensible heating or cooling.
Dry-bulb temperature: The ordinary air temperature measured by a thermometer shielded from radiation and moisture exchange. It is the temperature coordinate from which other moist-air properties are interpreted.
Air-conditioning: The control of indoor air temperature, humidity, cleanliness, and circulation. Cooling coils and dehumidifiers are analyzed as changes in moist-air state.
Absolute humidity: The mass of water vapor per unit volume of air. Unlike humidity ratio, it changes with air volume and density.
Relative humidity: The ratio of water vapor partial pressure to saturation vapor pressure at the same temperature. It varies with temperature even when the air's moisture content stays constant.
Dalton's law of partial pressures: A gas-mixture law stating that total pressure equals the sum of component gases' partial pressures. It separates moist-air pressure into contributions from dry air and water vapor.