def make_hour_spec(start_hour):
    """Return an hour_spec given the start_hour.

    Ex. 0 -> H_0000_2000, 22 -> H_2200_0000
    """
    if not (0 <= start_hour <= 22):
        raise ValueError("start hour must be between 0 and 22 inclusive")
    elif start_hour % 2 != 0:
        raise ValueError("must start on an even hour")

    end_hour = (start_hour + 2) % 24
    return "_".join(["H", "%02d00" % start_hour, "%02d00" % end_hour])