Compute psychrometric constant [kPa / C] as described in the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper 56 entitled:

Crop evapotranspiration - Guidelines for computing crop water requirement. Specifically, see equation 7 of Chapter 3 or equation 3-2 in Annex 3.

From FAO 56:

The specific heat at constant pressure is the amount of energy required to increase the temperature of a unit mass of air by one degree at constant pressure. Its value depends on the composition of the air, i.e., on its humidity. For average atmospheric conditions a value cp = 1.013 10-3 MJ kg-1 C-1 can be used. As an average atmospheric pressure is used for each location (Equation 7), the psychrometric constant is kept constant for each location.

A table listing the psychrometric constant for different altitudes is located here:


pressure (numpy.ndarray, xarray.DataArray, list, float) – pressure in kPa/C


psy_const – the computed psychrometric constant. Same shape as pressure.

Return type

numpy.ndarray, xarray.DataArray


>>> import numpy as np
>>> from geocat.comp import psychrometric_constant
>>> pressure = np.array([60, 80, 100])
>>> psychrometric_constant(pressure)
array([0.0398844, 0.0531792, 0.066474 ])

See also

Related NCL Functions: psychro_fao56