def tropical_nights(tasmin, thresh='20.0 degC', freq='YS'):
    r"""Tropical nights

    The number of days with minimum daily temperature above threshold.

    Parameters
    ----------
    tasmin : xarray.DataArray
      Minimum daily temperature [℃] or [K]
    thresh : str
      Threshold temperature on which to base evaluation [℃] or [K]. Default: '20 degC'.
    freq : str, optional
      Resampling frequency

    Returns
    -------
    xarray.DataArray
      Number of days with minimum daily temperature above threshold.

    Notes
    -----
    Let :math:`TN_{ij}` be the daily minimum temperature at day :math:`i` of period :math:`j`. Then
    counted is the number of days where:

    .. math::

        TN_{ij} > Threshold [℃]
    """
    thresh = utils.convert_units_to(thresh, tasmin)
    return tasmin.pipe(lambda x: (tasmin > thresh) * 1) \
        .resample(time=freq) \
        .sum(dim='time')