Calculate precipitable water from surface dew point temp (Perez eqn 4),
    or return a default value for use with :py:func:`_dirint_bins`.

def _temp_dew_dirint(temp_dew, times):
    """
    Calculate precipitable water from surface dew point temp (Perez eqn 4),
    or return a default value for use with :py:func:`_dirint_bins`.
    """
    if temp_dew is not None:
        # Perez eqn 4
        w = pd.Series(np.exp(0.07 * temp_dew - 0.075), index=times)
    else:
        # do not change unless also modifying _dirint_bins
        w = pd.Series(-1, index=times)
    return w