# psychrometric_vapor_pressure_wet#

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].

Parameters:
Returns:

pint.Quantity – Vapor pressure

Examples

>>> 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


Notes

$e' = e'_w(T_w) - A p (T - T_w)$
• $$e'$$ 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)