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.
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:
{Reference: FAAO 7110.65 2-6-4}
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:
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:
{Reference: FAAO 7110.65 2-7}
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.