Magnus equation
An empirical equation that estimates saturation vapor pressure from temperature. Magnus-form coefficients are fitted to measured vapor-pressure data.
The Magnus equation is a family of empirical formulas used to estimate saturation vapor pressure Saturation vapor pressure (SVP) The equilibrium pressure of water vapor over a flat water surface at a given temperature. It rises nonlinearly as temperature increases. from temperature. It approximates measured behavior rather than deriving vapor pressure directly from first principles.
One commonly used form over liquid water is SVP = 0.61094 × exp((17.625 × T) / (T + 243.04)). T is in °C and the result is in kilopascals. Different coefficient sets exist for different reference data and temperature ranges, so calculations should state the form used.
The coefficients above are associated with the Alduchov and Eskridge refit and closely approximate standard reference values from -40 to 140 °F (-40 to 60 °C). The equation can support conversions among relative humidity Relative humidity The share of moisture air is holding against the most it could hold at that temperature. Because it moves with temperature, cooling air raises RH with no water added. , vapor pressure deficit Vapor pressure deficit The difference between saturation vapor pressure and actual water-vapor pressure. Leaf-to-air VPD is one driver of transpiration, moderated by stomata and water supply. , and dew point Dew point The temperature at which air with its current water-vapor content reaches saturation. Cooling a surface to or below the dew point can produce condensation. when the necessary inputs are known.