Relative humidity gives the ratio between the actual water vapour partial pressure and the saturation vapour pressure at the same temperature. Everything else follows from that one quantity.

Saturation vapour pressure

p_ws(T) uses the Arden Buck (1981) correlation over liquid water. It agrees with the IAPWS-IF97 steam tables to within 0.05 % between -20 and +60 °C. The dew point is obtained by inverting the same correlation numerically, so dew point and vapour pressure stay mutually consistent.

Dew point and frost point

Below 0 °C the calculator also reports the frost point, from the Buck correlation over ice. Ice has a lower saturation vapour pressure than supercooled water, so water deposits as frost at a higher surface temperature than the dew point suggests. The gap widens as the gas gets drier: about 1 °C at a dew point of -10 °C and about 3.5 °C at -40 °C. In that range the frost point, not the dew point, sets the minimum surface temperature, and the surface check uses it.

Basis and reference state

ppmv is reported on a wet basis, as a fraction of the total pressure. Some calculators report the dry basis, p_w/(p − p_w), which is higher by roughly 1.3 % at 23 °C and 45 % RH and by much more in humid gas. Absolute humidity is the water vapour density at the entered temperature and pressure, not at normal conditions. The humidity ratio and ppmw depend on the carrier gas: dry air at 28.9647 g/mol is the default, and switching to nitrogen at 28.0134 g/mol raises the humidity ratio by about 3.4 % for the same vapour pressure.

Simplifications

Water vapour and the carrier gas are treated as ideal gases, R_v = 461.52 J/(kg·K). The water vapour enhancement factor, which accounts for the presence of the carrier gas and adds roughly 0.4 % to saturation vapour pressure at ambient conditions, is not applied. Neither is real gas compressibility. Both are below the uncertainty of a typical process humidity measurement at atmospheric pressure, and both grow at elevated pressure. Enthalpy is not calculated, since it is only well defined for moist air.