import math


def calc_saturated_vapour_pressure_air_FAO(temp_air):
    """Saturated vapour pressure of air at temp_air in kPa

    From: http://www.fao.org/3/X0490E/x0490e0k.htm
    """
    return 0.611 * math.exp((17.27 * temp_air) / (temp_air + 237.3))