def getab(taub, taud):
    """calculates value of ab in equation (19)

    Equation (19) in ASHRAE Fundamentals 2009 pg 14.9

    Parameters
    ----------
    taub : float
        value of taub in equation (19)
    taud : float
        value of taud in equation (19)

    Returns
    -------
    float
        the value of ab

    """
    v1 = 1.219
    v2 = 0.043 * taub
    v3 = 0.151 * taud
    v4 = 0.204 * taub * taud
    return v1 - v2 - v3 - v4
