General ATC — Weather Last updated: 2017-07-10

Reading METAR format


Weather reports are issued in the METAR format. To learn more about reading this format, see the article “Ok, I've read the METAR, now what does all that mean?” on the ZLA home page. Numerous sites can also be found online to decode METAR abbreviations.

Pilot Weather Deviations


Due to thunderstorms, or other conditions, a pilot may wish to deviate from his cleared route. If conditions permit, approve these deviations using phraseology like:

  • “UAL43, deviation approved, when able proceed direct LAS.”

  • “N34S, deviation right of course approved, when able, fly heading 220, vector ILS runway 25L approach.”

  • “VVHK562, deviation up to twenty degrees left of course approved, when able, rejoin J501.”

{Reference: FAAO 7110.65 2-6-4}

Altimeter Settings


When below FL180, pilots are required to set their altimeter(s) to the setting of a station within 100 miles of the aircraft. Issue altimeter settings as follows:

  • 1.Departures – Must be issued at least once to each departure aircraft. Normally, this should be accomplished by the first controller to communicate with the aircraft. For example, if both Ground and Tower positions are open, the ground controller must issue the altimeter setting. If a pilot advises that he has the latest ATIS for the airport, then the altimeter setting need not be issued.

  • 2.Arrivals – Must be issued by the controller who first communicates with the aircraft about its arrival. If a pilot advises that he has the latest ATIS for the airport, then the altimeter setting need not be issued.

  • 3.En route – When operating below FL180, an aircraft must be issued an altimeter setting at least once while in your airspace.

  • 4.Descending below FL180 – When issuing a descent clearance below FL180, issue an altimeter setting for the weather station nearest the point where the aircraft will descend through FL180.

Arrivals and departures need not be advised of the source of the altimeter setting if it is for their arrival or departure airport. For all other aircraft, when issuing an altimeter setting, always identify the station referenced. use phraseology like:

  • “The Los Angeles altimeter three zero four five.”
  • For an aircraft arriving or departing Los Angeles: “altimeter three zero four five.”

{Reference: FAAO 7110.65 2-7}

Runway RVR (Runway Visibility Range) values


In the RW, RVR values are the most accurate measure of visibility for a pilot. It is specific to a runway, and is the determining factor in whether or not a pilot has the minimum visibility to attempt an approach, or depart an airport. RW controllers have a continuously updated display of RVR values for all runways at an airport that they can use to advise pilots. Unfortunately, on vatsim we have no such display, and pilot's flight simulators do not model the data.

In the RW, the issuance of RVR data is mandatory, since it is controlling for most operators. On Vatsim, METAR reports will often include RVR data when visibility is less than 1 mile. The issuance of that data is optional, since it is not dynamic, not updated frequently, and may not be modeled in the pilot's flight simulator software.

Back to Top