Calculate the d coefficient.

def _calc_d(aod700, p):
    """Calculate the d coefficient."""

    p0 = 101325.
    dp = 1/(18 + 152*aod700)
    d = -0.337*aod700**2 + 0.63*aod700 + 0.116 + dp*np.log(p/p0)

    return d