def get_parameters():
    """
    retrieve a list of parameter codes available.
    Reference for GSOD parameters : https://www1.ncdc.noaa.gov/pub/data/gsod/readme.txt

    Parameters
    ----------
        None

    Returns
    -------
        dictionary of variables with parameter codes as keys
        and GSOD codes as values.
    """
    VARIABLES = {'mean_temp': 'TEMP', 'mean_temp_count': 'TEMP',
        'dew_point': 'DEWP', 'dew_point_count': 'DEWP',
        'sea_level_pressure': 'SLP     ', 'sea_level_pressure_count':
        'SLP     ', 'station_pressure': 'STP', 'station_pressure_count':
        'STP', 'visibility': 'VISIB', 'visibility_count': 'VISIB',
        'mean_wind_speed': 'WDSP', 'mean_wind_speed_count': 'WDSP',
        'max_wind_speed': 'MXSPD', 'max_gust': 'GUST', 'max_temp': 'MAX',
        'max_temp_flag': 'MAX', 'min_temp': 'MIN', 'min_temp_flag': 'MIN',
        'precip': 'PRCP', 'precip_flag': 'PRCP', 'snow_depth': 'SNDP',
        'FRSHTT': 'FRSHTT'}
    return VARIABLES
