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:

• 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.

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

See also

saturation_vapor_pressure

Notes

$$e^{\prime }=e_w^{\prime }(T_w)-Ap(T-T_w)$$

• $e^{\prime }$ is vapor pressure

• $e_w^{\prime }(T_w)$ 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)