def relative_humidity(e_a, e_s):
    """
    Calculate the relative humidity of air (Allen et al., 1998).

    Relative humidity expresses the degree of saturation of the air as a ratio
    of the actual (e_a) to the saturation (e_s) vapour pressure at the same
    temperature.

    Parameters
    ----------
    e_a : float
        Actual vapour pressure at a given temperature (kPa).
    e_s : float
        Saturation vapour pressure at a given temperature (kPa).

    Returns
    -------
    float
        Relative humidity (%).

    """
    return (e_a / e_s) * 100