import math


def avp_from_tdew(tdew):
    """
    Estimate actual vapour pressure (*ea*) from dewpoint temperature.

    Based on equation 14 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.

    :param tdew: Dewpoint temperature [deg C]
    :return: Actual vapour pressure [kPa]
    :rtype: float
    """
    return 0.6108 * math.exp((17.27 * tdew) / (tdew + 237.3))