Aviation Weather

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Aviation Weather Theory

The pilot's weather tools

To accurately gather weather data to ensure a successful flight, pilots have access to many different resources that inform them of what the weather is up to.

METARs

Every pilot and even student for that matter has been exposed to a METAR. Roughly translated from the French as METeorlogical Aviation Routine weather Reports, a METAR is the hourly surface weather observation issued 5 minutes before the hour. It is available to the aviation community and used by the National Weather Service to determine an airport's flying conditions (IFR, MVFR, VFR). You can obtain these from a variety of places both online and by phone.

This is the basic knowledge of what ground school consists of on this topic other than deciphering its acronyms. Although as a pilot, sometimes its nice to know a little about how these services came to be without going too much into detail. Funny enough, the change to our current acronym of METAR is fairly recent.

History

Before the current interpretation of weather data, there were two formats, and, in rough terms, it was "us and them" or rather, to be more precise, the North American countries reported weather differently from the rest of the world. North Americans were using SAO or "Surface Aviation Observation" (which was adopted in the 1950s), while the rest of the world was using the currently known METAR or "METeorological Aviation Routine weather reports". The FAA, which determines aviation requirements in the US, became increasingly aware that expanding numbers of international flights and pilots alike was creating a strong need to standardize weather report interpretations internationally.

The National Weather Service standardized the weather reports into what we now know as METARs. To reduce the stress on US aviation citizens, the metric system was kept to a minimum: for example, the winds were kept in knots instead of meters per second, visibility in miles, altimeter settings in inches of mercury instead of hectopascals (hPa), and RVR continued in feet. Temperatures, however, are converted to Celsius to allow for better conversions. The biggest change is simply the order in which elements are reported.

Translating METARs

So in a sense, although the attempt at standardizing weather code from SAO to METAR is apparent, the actual translation is not substantial. Oh and for those of you who are students or pilots that don’t like translating code, there is a reason and a cure. First off do realize that METARs are always originated in code and probably will be for a while. The reason seems to be that with the vast amount of changes and updates to weather reports would overload the system. The great news is that now it is possible to translate the code online either through request from sites such as NOAA or by looking up the code yourself which will put it in simple English. Sometimes this helps those who are new to learn the code and not miss anything during preflight weather preparations.

A closer look at the METAR

The following example is a METAR taken from Vancouver International Airport (CYVR) in Vancouver, British Columbia:

METAR CYVR 120200Z 14021G27KT 20SM -SHRA FEW030 BKN058 OVC090 10/06 A2982 RMK SC2SC5AC2 PCPN VRY LGT SLP098=

Like mentioned above, METARs are observed and posted on an hourly basis. An airport can also issue an METAR that is not on the hour, this observation is called a SPECI. SPECI is abbreviated for special because the report was issued based on a significant change in conditions such as a violent temperature change, cloud layer movement, precipitation, visibility, etc. Looking back at the Vancouver METAR, the observation can be translated as such.

METAR: Simply indicated that the observation is a METAR observation.

CYVR: This is the aerodrome ident of which the observation originates, in this case, the aerodrome is Vancouver International.

120200Z: This is the time of which the METAR was issued, the first two numbers are the date, so 12 would mean the twelfth day of the month. The 0200Z is the time indicated in UTC, in aviation, it is indicated as Zulu time hence the Z on the end. The time 0200 can be translated to 2:00.

14021G27: This is the current wind observation. The first three numbers indicate the wind direction in degrees true while the following numbers indicate the wind speed measured in knots. From this figure we gather that the wind is at 140 degrees true and is at 21 knots gusting to 27 knots.

20SM: Indicates the visibility at the airport; in this case, the visibility is 20 Statute Miles.

-SHRA: Means that there is rain showers at the airport, the negative sign at the beginning indicates the severity of the precipitation, a negative sign meaning light, no sign at all meaning moderate, a positive(+) sign meaning heavy.

FEW030 BKN058 OVC090: These are the current cloud layers observed. FEW030 is few clouds at 3000 feet ASL, BKN058 is broken clouds at 5800 feet ASL, and OVC090 is overcast clouds at 9000 feet ASL. ASL is abbreviated for above sea level, and you add two zeros on the end of the numbers to receive the actual altitude of the clouds.

10/06: Current temperature and dewpoint, Temperature is 10 degrees celsius while the dewpoint is 6 degrees celsius.

A2982: Altimeter setting observed, in this case 29.82 Hg/m

RMK SC2SC5AC2 PCPN VRY LGT: Any remarks which the observation has. SC2SC5AC2 indicates that in respect to the observed cloud layers, the clouds are composed of 2 ocatas of strato cumulus clouds, 5 octas of stratocumulus clouds, and 2 octas of alto-cumulus clouds. PCPN VRY LGT indicates that precipitation is very light.

SLP098: Indicates sea level pressure.

Quiz

1 METARs are roughly translated from the French as _________.

	METeorological Aviation Routine weather Reports
	METeorological Airline Regular weather Reports
	METeorological Airport Regular weather Reports

2 Before the current interpretation of weather data, North Americans were using _________ while the rest of the world was using METAR.

	SAO or Surface Aviation Observation
	Surface Aircraft Observation
	Standard Aviation Observation

3 The FAA became increasingly aware that expanding numbers of international flights and pilots was creating a need to _________.

	standardize weather report interpretations internationally
	localize weather report interpretations
	diversify weather report interpretations

4 METARs are always originated in code because _________.

	the vast amount of changes and updates to weather reports would overload the system
	it's easier for pilots to read
	it's an international standard

5 In the METAR example from Vancouver International Airport, "14021G27" indicates _________.

	the wind is at 140 degrees true and is at 21 knots gusting to 27 knots
	the wind is at 140 knots and is gusting from 21 to 27 degrees
	the wind is at 21 degrees true and is at 140 knots gusting to 27 knots

6 In the METAR example, "FEW030 BKN058 OVC090" refers to _________.

	the current cloud layers observed
	the visibility range in miles
	the temperature and dewpoint

Resources

• http://www.nws.noaa.gov/oso/oso1/oso12/overview.htm - National Weather Services
• http://www.alaska.faa.gov/fai/afss/metar%20taf/metintro.htm (Also a good source for additional info and abbreviation translations.)
• http://www.nwstc.noaa.gov/METEOR/AvnOps/aoc_webpage.htm NOAA National Weather Service Training Center.
• http://adds.aviationweather.noaa.gov Aviaition Digital Data Service Aviation Weather Center