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.
Saturation vapor pressure is the water-vapor pressure at equilibrium with liquid water at a specified temperature. If air reaches that vapor pressure, it is saturated. The value depends strongly on temperature and is commonly expressed in kilopascals.
As temperature rises, saturation vapor pressure rises nonlinearly. 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. compares the actual vapor pressure with this saturation value, while 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. describes the difference between them.
The Magnus equation Magnus equation An empirical equation that estimates saturation vapor pressure from temperature. Magnus-form coefficients are fitted to measured vapor-pressure data. is one empirical way to estimate saturation vapor pressure from temperature. When air cools until its actual vapor pressure equals the saturation value, it reaches the 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. , and condensation can begin on surfaces at or below that temperature.