def exner_function(pressure, reference_pressure=mpconsts.P0):
    r"""Calculate the Exner function.

    .. math:: \Pi = \left( \frac{p}{p_0} \right)^\kappa

    This can be used to calculate potential temperature from temperature (and visa-versa),
    since

    .. math:: \Pi = \frac{T}{\theta}

    Parameters
    ----------
    pressure : `pint.Quantity`
        The total atmospheric pressure
    reference_pressure : `pint.Quantity`, optional
        The reference pressure against which to calculate the Exner function, defaults to P0

    Returns
    -------
    `pint.Quantity`
        The value of the Exner function at the given pressure

    See Also
    --------
    potential_temperature
    temperature_from_potential_temperature

    """
    return (pressure / reference_pressure).to('dimensionless')**mpconsts.kappa