def METHOD_NAME(df):
    """ Turn weather_date into datetime type then create 3 columns, year, month, and day """
    df['weather_date'] = df['weather_date'].apply(str)
    df['year'] = df['weather_date'].apply(lambda x: int(x[:4]))
    df['month'] = df['weather_date'].apply(lambda x: int(x[4:6]))
    df['day'] = df['weather_date'].apply(lambda x: int(x[6:]))