def calc_ndsi(green, swir1):
    """NDSI calculation
    normalized difference snow index

    Parameters
    ----------
    green: ndarray
    swir1: ndarray
        ~1.62 µm
        Band 6 in Landsat 8

    Output
    ------
    ndarray:
        unitless index
    """
    return (green - swir1) / (green + swir1)
