content. For the objective elements, such as pressure,  ambient temperature, dew point temperature, wind, and  precipitation accumulation, both the automated system  and the observer use a fixed location and time-averaging  technique. The quantitative differences between the  observer and the automated observation of these elements  are negligible. For the subjective elements; however,  observers use a fixed time (spatial averaging technique) to  describe the visual elements (sky condition, visibility, and  present weather, etc.), while the automated systems use  a fixed location and time averaging technique. Although  this is a fundamental change, the manual and automated  techniques yield remarkably similar results within the limits  of their respective capabilities.  The use of the aforementioned visibility reports and  weather services are not limited for Part 91 operators.  Part 121 and 135 operators are bound by their individual  OpSpecs documents and are required to use weather  reports that come from the NWS or other approved  sources. While all OpSpecs are individually tailored, most  operators are required to use ATIS information, runway  visual range (RVR) reports, and selected reports from  automated weather stations. All reports coming from an  AWOS-3 station are usable for Part 121 and 135 operators.  Each type of automated station has different levels of  approval as outlined in individual OpSpecs. Ceiling and  visibility reports given by the tower with the departure  information are always considered official weather, and  RVR reports are typically the controlling visibility reference.  Refer to Chapter 1, Departures, of this manual, as well as  the AIM section 7-1-12 for further description of automated  weather systems.  Center weather advisories (CWAs) are unscheduled inflight,  flow control, air traffic, and aircrew advisories. By nature of  its short lead time, the CWA is not a flight planning product.  It is generally a nowcast for conditions beginning in the  next two hours. CWAs will be issued:  For flight planning purposes, weather information must be  reviewed in order to determine the necessity and suitability  of alternate airports. For Part 91 operations, the 600-2 and  800-2 rule applies to airports with precision and non- precision approaches, respectively. Approaches with  vertical guidance (APV) are non-precision approaches  because they do not meet the International Civil Aviation  Organization (ICAO) Annex 10 standards for a precision  approach. (See Final Approach Segment section later in this  chapter for more information regarding APV approaches.)  Exceptions to the 600-2 and 800-2 alternate minimums are  listed in the front of the Aeronautical Information Services  in the Terminal Procedures Publication (TPP) and are  indicated by a symbol  A  on the approach charts for the  airport. This does not preclude flight crews from initiating  instrument approaches at alternate airports when the  weather conditions are below these minimums. The 600­ 2 and 800-2 rules, or any exceptions, only apply to flight  planning purposes, while published landing minimums  apply to the actual approach at the alternate.  According to 14 CFR Part 91, § 91.103, the pilot in command  (PIC) must become familiar with all available information  concerning a flight prior to departure. Included in this  directive is the fundamental basis for pilots to review  NOTAMs and pertinent weather reports and forecasts  for the intended route of flight. This review should  include current weather reports and terminal forecasts  for all intended points of landing and alternate airports.  In addition, a thorough review of an airport’s current  weather conditions should always be conducted prior  to initiating an instrument approach. Pilots should also  consider weather information as a planning tool for fuel  management.  There are many practical reasons for reviewing weather  information prior to initiating an instrument approach.  Pilots must familiarize themselves with the condition of  individual airports and runways so that they may make  informed decisions regarding fuel management, diversions,  and alternate planning. Because this information is critical,  14 CFR requires pilots to comply with specific weather  minimums for planning and execution of instrument flights  and approaches..  weather information sources indicate that existing  or anticipated meteorological phenomena will  adversely affect the safe and efficient flow of air  traffic within the ARTCC area of responsibility.  Weather Regulatory Requirements  Weather Requirements and Part 91 Operators  Center Weather Advisories (CWA)  1. As a supplement to an existing SIGMET, convective  SIGMET, or AIRMET.  2. When an in-flight advisory has not been issued  but observed or expected weather conditions  meet SIGMET/AIRMET criteria based on current  pilot reports and reinforced by other sources  of information about existing meteorological  conditions.  3. When observed or developing weather conditions  do not meet SIGMET, convective SIGMET, or  AIRMET criteria (e.g., in terms of intensity or area  coverage), but current pilot reports or other  4-6 