def climatology_means(series, climatology_years):
    """Given a series of monthly data (for one or more month values) gather each
    month's data (Jan, Feb, etc) that falls within the range
    climatology_years and compute the mean of those items.

    Return a series of (month, mean values)

    """
    s = series.copy()
    clim_year_series = s[(s.index.year >= climatology_years[0]) &
                         (s.index.year <= climatology_years[1])]

    climatology_means_by_month = clim_year_series.groupby(clim_year_series.index.month).mean()

    return climatology_means_by_month