def rh_from_avp_svp(avp, sat_vp):
    """
    Calculate relative humidity as the ratio of actual vapour pressure
    to saturation vapour pressure at the same temperature.

    See Allen et al (1998), page 67 for details.

    :param avp: Actual vapour pressure [units do not matter so long as they
        are the same as for *svp*]. Can be estimated using functions whose
        name begins with 'avp_from'.
    :param sat_vp: Saturated vapour pressure [units do not matter so long as they
        are the same as for *avp*]. Can be estimated using ``svp_from_t()``.
    :return: Relative humidity [%].
    :rtype: float
    """
    return 100.0 * avp / sat_vp