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Humid air is a mixture of dry air and water vapor; key properties are dry-bulb temperature, wet-bulb temperature, relative humidity, dew point, and humidity ratio. The psychrometric chart plots these interrelationships and enables rapid analysis of HVAC processes: heating, cooling, humidification, and dehumidification. Accurate calculations use ideal-gas approximations and water saturation properties.
From your work with Dalton's law and gas mixtures, you know that the total pressure of a gas mixture equals the sum of each component's partial pressure. Humid air is a binary mixture: dry air and water vapor, each contributing its partial pressure to the total atmospheric pressure (~101.3 kPa). Psychrometrics is the application of mixture thermodynamics to this specific, practically ubiquitous mixture — the air you breathe, condition, and work with in HVAC and industrial processes.
The most important quantity is the humidity ratio ω (also called specific humidity): the mass of water vapor per kilogram of dry air. It determines how much moisture the air actually carries. Related but different is relative humidity φ: the ratio of the actual partial pressure of water vapor to the saturation pressure at the current temperature. When φ = 100%, the air is saturated — it is holding all the water vapor it can at that temperature. The dew point is the temperature at which your air would reach saturation if cooled at constant pressure; below this temperature, water condenses out.
The wet-bulb temperature (measured by a thermometer with a wet wick exposed to airflow) is lower than the dry-bulb temperature (ordinary thermometer) because evaporation from the wick cools it. The wet-bulb depression (dry-bulb minus wet-bulb) is proportional to how dry the air is — desert air shows a large depression; saturated air shows zero depression because no evaporation occurs. These two temperatures together uniquely specify the state of humid air, which is why standard weather reports often give them. The psychrometric chart encodes all five of these properties simultaneously: fixing any two of {dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio} determines the rest.
Tracing HVAC processes on the psychrometric chart makes the physics visual. Sensible heating (turning on a furnace with no added moisture) moves horizontally rightward — temperature rises, but humidity ratio is unchanged. Cooling below the dew point moves along the saturation curve as moisture condenses out — this is how air conditioners dehumidify. Humidification (adding steam) moves upward — humidity ratio increases at roughly constant temperature. Adiabatic saturation (spraying water into airflow) moves along lines of constant wet-bulb temperature toward saturation. Each of these processes involves an energy balance on the air-water system, and the enthalpy values read from the chart let you calculate heating and cooling loads directly.