WMO-No. 8 humidity equations
Dew Point over Water
Calculate thermodynamic dew-point temperature from dry-bulb temperature and relative humidity with respect to water. The WMO water and ice formula ranges are enforced—no extrapolated result is shown.
WMO-range result
WMO-No. 8, Guide to Instruments and Methods of Observation, Volume I, Chapter 4, Annex 4.B gives ew(t)=6.112 exp[17.62t/(243.12+t)] for water from −45 to 60°C and ei(t)=6.112 exp[22.46t/(272.62+t)] for ice from −65 to 0°C. WMO e-Library chapter viewer.
Den WMO International Cloud Atlas glossary defines dew point with respect to a water surface and relative humidity as observed vapour pressure divided by saturation vapour pressure at the same temperature and pressure.
Corrections to the former calculator: t−td is dew-point depression, not wet-bulb depression. Wet-bulb temperature is a separate pressure-dependent psychrometric quantity. The former vapour-pressure result mixed a second coefficient set and omitted the moist-air enhancement factor, so it has been removed. Comfort bands, presets and unsourced FAQ claims were also removed.
