Many different variables can be used as measures of water-vapor content in air, including: water-vapor partial pressure (known as vapor pressure in meteorology), mixing ratio, absolute humidity, specific humidity, relative humidity, dew-point temperature, lifting condensation level, and wet-bulb temperature. Some of these variables can be used to quantify total water (vapor + liquid + ice).
Formulas exist to convert between these different variables. Some of these humidity variables can be easily measured by instruments called hygrometers, others are useful in conservation equations, while others are commonly known by the general public.
The amount of water actually being held in the air might be less than the maximum amount that could be held at equilibrium, where this equilibrium value is known as the saturation humidity. Cooler air can hold less water vapor at saturation than warmer air — a fact that is critical in understanding why clouds and storms form in rising, cooling air.
By following such a rising or sinking air parcel we can write a Lagrangian moisture budget and calculate adiabatic temperature changes for air that is saturated (foggy or cloudy). A graphical description of this process can be represented as moist adiabats on a thermo diagram. Saturated rising air does not cool as rapidly with altitude as dry air. Thermo diagrams can also include isohume lines that relate humidity state to temperature and pressure.
An alternative frame of reference is Eulerian, which is fixed relative to a location on the Earth’s surface. To forecast humidity in such a fixed frame, we can account for the advection of moisture by the winds, the fluxes of moisture due to turbulence, and effects due to precipitation. Clouds and precipitation are discussed in subsequent chapters.