metpy.calc.psychrometric_vapor_pressure_wet(pressure, dry_bulb_temperature, wet_bulb_temperature, psychrometer_coefficient=None)[source]#

Calculate the vapor pressure with wet bulb and dry bulb temperatures.

This uses a psychrometric relationship as outlined in [WMO8], with coefficients from [Fan1987].

  • pressure (pint.Quantity) – Total atmospheric pressure

  • dry_bulb_temperature (pint.Quantity) – Dry bulb temperature

  • wet_bulb_temperature (pint.Quantity) – Wet bulb temperature

  • psychrometer_coefficient (pint.Quantity, optional) – Psychrometer coefficient. Defaults to 6.21e-4 K^-1.


pint.Quantity – Vapor pressure


>>> from metpy.calc import psychrometric_vapor_pressure_wet, saturation_vapor_pressure
>>> from metpy.units import units
>>> vp = psychrometric_vapor_pressure_wet(958 * units.hPa, 25 * units.degC,
...                                       12 * units.degC)
>>> print(f'Vapor Pressure: {vp:.2f}')
Vapor Pressure: 628.15 pascal
>>> rh = (vp / saturation_vapor_pressure(25 * units.degC)).to('percent')
>>> print(f'RH: {rh:.2f}')
RH: 19.83 percent


\[e' = e'_w(T_w) - A p (T - T_w)\]
  • \(e'\) is vapor pressure

  • \(e'_w(T_w)\) is the saturation vapor pressure with respect to water at temperature \(T_w\)

  • \(p\) is the pressure of the wet bulb

  • \(T\) is the temperature of the dry bulb

  • \(T_w\) is the temperature of the wet bulb

  • \(A\) is the psychrometer coefficient

Psychrometer coefficient depends on the specific instrument being used and the ventilation of the instrument.

Changed in version 1.0: Changed signature from (dry_bulb_temperature, wet_bulb_temperature, pressure, psychrometer_coefficient)