def _convert_freq_string_to_description(freq):
    """Get long description for frequency string."""

    if freq in ('H', '1H'):
        return 'hourly'

    if freq in ('D', '1D'):
        return 'daily'

    if freq in ('W', '1W', 'W-SUN'):
        return 'weekly'

    if freq in ('1M', '1MS', 'MS'):
        return 'monthly'

    if freq in ('1A', '1AS', '1BYS', 'A-DEC'):
        return 'yearly'

    raise ValueError("Unsupported frequency string '%r'" % freq)