Calculate the b coefficient.

def _calc_b(w, aod700):
    """Calculate the b coefficient."""

    b1 = 0.00925*aod700**2 + 0.0148*aod700 - 0.0172
    b0 = -0.7565*aod700**2 + 0.5057*aod700 + 0.4557

    b = b1 * np.log(w) + b0

    return b