def getad(taub, taud):
    """calculates value of ad in equation (20)

    Equation (20) in ASHRAE Fundamentals 2009 pg 14.9

    Parameters
    ----------
    taub : float
        value of taub in equation (20)
    taud : float
        value of taud in equation (20)

    Returns
    -------
    float
        the value of ad

    """
    v1 = 0.202
    v2 = 0.852 * taub
    v3 = 0.007 * taud
    v4 = 0.357 * taub * taud
    return v1 + v2 - v3 - v4
