import math


def avp_from_temperature_min(temperature_min):
    """
    Estimate actual vapour pressure (*ea*) from minimum temperature.

    This method is to be used where humidity data are lacking or are of
    questionable quality. The method assumes that the dewpoint temperature
    is approximately equal to the minimum temperature (*temperature_min*), i.e. the
    air is saturated with water vapour at *temperature_min*.

    **Note**: This assumption may not hold in arid/semi-arid areas.
    In these areas it may be better to subtract 2 deg C from the
    minimum temperature (see Annex 6 in FAO paper).

    Based on equation 48 in Allen et al (1998).

    :param temperature_min: Daily minimum temperature [deg C]
    :return: Actual vapour pressure [kPa]
    :rtype: float
    """
    return 0.611 * math.exp((17.27 * temperature_min) / (temperature_min + 237.3))