def avp_from_twet_tdry(twet, tdry, svp_twet, psy_const):
    """
    Estimate actual vapour pressure (*ea*) from wet and dry bulb temperature.
    Based on equation 15 in Allen et al (1998). As the dewpoint temperature
    is the temperature to which air needs to be cooled to make it saturated, the
    actual vapour pressure is the saturation vapour pressure at the dewpoint
    temperature.
    This method is preferable to calculating vapour pressure from
    minimum temperature.
    Values for the psychrometric constant of the psychrometer (*psy_const*)
    can be calculated using ``psyc_const_of_psychrometer()``.
    :param twet: Wet bulb temperature [deg C]
    :param tdry: Dry bulb temperature [deg C]
    :param svp_twet: Saturated vapour pressure at the wet bulb temperature
        [kPa]. Can be estimated using ``svp_from_t()``.
    :param psy_const: Psychrometric constant of the pyschrometer [kPa deg C-1].
        Can be estimated using ``psy_const()`` or
        ``psy_const_of_psychrometer()``.
    :return: Actual vapour pressure [kPa]
    :rtype: float
    """
    return svp_twet - (psy_const * (tdry - twet))