def get_average_temperature_during_heating_season(temperature, t_threshold=15):
    """
    returns average temperature during heating season
    input:
        temperature : pd.Series(Index=time, values=temperature)
        t_threshold : threshold temperature for heating degree days (HDD)
    returns:
        average temperature
    """
    t_average_daily = temperature.resample("1D").mean()
    return t_average_daily.loc[t_average_daily < t_threshold].mean()