The following description is based on Canadian metars. American metars are much the same but do have a few differences…
Metar.. is basically an aviation surface weather report and is used by pilots and meteorologists around the world. This is a standardized report and based on the ICAO format. The metar is not a forecast but rather an observation of current weather conditions observed at the station at the time of observation. The report emphasizes information of particular interest to aviation and presented in a standard format. The metar is issued every hour. If significant weather conditions change between hours then a special weather report is issued or a “SPECI” There is a standard set of parameters that require the issuance of a “speci” but basically issued when significant weather conditions change from the previously issued report whether it is a regular metar or speci.
So lets get into decoding a metar/speci.
Here is a metar issued for Charlottetown, PEI
CYYG 161800Z 04004KT 5SM -RA BR BKN003 OVC058 02/02 A2991 RMK SF5NS3 SLP131
The first group is the station identifier. 4 letter ICAO airport. In this case CYYG stands for Charlottetown.
Second group is the date and time. First 2 numbers are the date followed by the time in UTC of the report. In this case 16th at 1800z, If a speci is issued then the time of the speci would be given, (ie 161815z)
The next group is a 5 or 7 digit group and represents wind. The first 3 digits denotes wind direction and are rounded to the nearest 10 degrees and always given in degrees true. The speed is indicated the last 2 and is reported using knots abbreviated KT. If the wind is gusting then a “G’ is reported as well and the speed of the gust is given. If a squall occurs then the letter Q is used followed by the speed of the squall. Definition of a gust is a sudden but brief increase in wind speed and must be at least 5kts greater than the mean. IE 15015G20kt which is 150 degrees true at 15 knots gusting to 20 kts. Calm wind is reported as 00000 kt
The next group represents prevailing visibility and reported in Statue miles abbreviated SM.
The next group represents RVR values if given. RVR stands for Runway visual range and is a determined by using 2 sensors placed a strategic points along the runway edge. One is a transmitter and the other is a receiver. The receiver calculates the strength of the signal and deteriorates with the presence of obstructions such as fog, snow, rain, etc in the air. The value is reported in feet once visibility drops below 6000. Here is a metar with an RVR value included. CYHZ 161800Z CCA 01008KT 1SM R23/P6000FT/U -RA BR OVC001 04/03 A2984 RMK ST8 /R10/ SLP109.
The R stands for RVR, The next number represents the applicable runway. In this case it is Runway 23. The next number is the RVR value and in this case is p6000ft or plus 6000ft or better than 6000ft. The last letter indicates the trend. U upward trend, N no change, D downward trend. A particular airport may have 2 or 3 RVRs reported .
The nest group represents weather conditions or significant weather. There is a list available on the abbreviations used.
|1 – Intensity or proximity||2 –
|– LightModerate (no qualifier)+ Heavy (well-developed in the case of dust/sand whirls, dust devils and funnel clouds)VC In the Vicinity||MI ShallowBC PatchesPR Partial (covering part of the aerodrome)DR Low Drifting
FZ Freezing (supercooled)
|DZ DrizzleRA RainSN SnowSG Snow Grains
IC Ice Crystals (diamond dust)
PL Ice Pellets
GS Small Hail and/or Snow Pellets
UP Unknown Precipitation (AWOS only)
|BR MistFG FogFU SmokeVAVolcanic Ash
DU Widespread Dust
|PO Dust/Sand Whirls (dust devils)SQ SquallsFC Funnel Cloud(s) (tornado or water-spout)SS Sandstorm
Here are a 4 metars.
CYYG 161900Z 01006KT 5SM -RA BR BKN003 OVC030 01/01 A2993 RMK ST7NS1 SLP137
CYQM 161900Z 02004KT 5SM -SHRA BR OVC003 02/02 A2989 RMK SF8 PRESRR SLP126
KBGR 161853Z 03004KT 10SM OVC012 03/00 A2989 RMK AO2 SLP125 T00280000
KBOS 161854Z 04005KT 1/4SM R04R/2200V3000FT FG OVC003 03/02 A2985 RMK AO2 SFC VIS 1/2 SLP107 T00330022
CYYG is reporting visibility of 5 miles in light rain and mist, CYQM is reporting visibility of 5 miles in light rainshowers and mist, KBGR reporting visibility of 10 miles with no significant weather, KBOS is reporting visibility of 1/4 mile in fog.. The v in the RVR indicated that the rvr value is variable or fluctuating. In this case 2000ft variable 3000ft
The next group represents sky conditions and is represented in 8ths or octaves. Sky condition first. sky clear.. CLR, 1-2 octaves.. FEW, 3-4 Octaves.. SCT, 5-7 Octaves… BKN, 8 octaves..OVC. The numbers immediately following the amount represents cloud height in feet and is reported in AGL or above ground level. ie SCT010 would be scattered at 1000ft, BKN025 broken at 2500ft, OVC200 would be overcast at 20,000ft. The cloud height is always reported as a 3 figure group and several layers are often reported. If the sky condition is obscured by a surface based layer such as fog, a blowing condition, or falling precipitation then the height that you can see directly straight up through this layer is reported and is represented by the letters VV which stand for vertical visibility. The next 3 numbers would be the height. VV005 indicates vertical visibility of 500 hundred feet. Think of a pilot trying to land his aircraft during a heavy snow event. The aircraft is actuallyflying below the base of the clouds but still cant’s see the ground directly below him due the falling snow. The vertical visibility would give the pilot an idea of when he can expect to see ground contact. The heavier the precipitation then the lower this value would be. Fog is the same.. if you think of a thick fog that completes conceals the overlying clouds. As the pilot descends into this fog then the vertical visibility would tell him when to expect to see the ground.
Here is a metar for CYYT. Note the VV used in the sky condition.
CYYT 161900Z 29005KT 1/4SM R16/1400FT/N R29/1600V2200FT/N -DZ FG VV001 06/06 A2999 RMK FG8 SLP162
In this case it is vertical visibility of 100ft.
The next group is the temperature and dew point group and is reported in Celsius. 2 digits each and is whole degrees, ie temperatures are rounded. In the example above, the temperature is 6 and dew point is 6. Temperatures below zero would have a “M” in front. ie M12/M15. Sometimes you may see M00. This would indicate a rounded value of zero but actually below zero. A temperatures of -0.2c and dew point of -0.4c would be reported as M00/M00
The next group is the altimeter setting. The altimeter is an instrument on board an aircraft which basically is a barometer and measures height above sea level. When this instrument is set to the current altimeter of the airport then the actual height of the aircraft is above sea level is displayed. This instruments works on the principle that atmospheric pressure falls with height. If you were to take your home barometer that you have on your wall up in an aircraft then the barometric pressure would fall accordingly indicating lower pressure. Same thing for the altimeter on the aircraft but reads height in feet instead of MB. The altimeter has to be updated from place to place because of high and low pressure systems that occur on the surface. When an aircraft takes-off then it is climbing into areas of lower pressure. This lower pressure is displayed as a height above sea level. When an aircraft descends then it is dropping into regions of higher atmospheric pressure hence the altimeter reading displays this new measurement as a lower altitude.
The next group is remarks… which include cloud type and amounts, anything of significance, etc. The American metars use the start and end time of precipitation here as well.
KDET 161620Z 19016G22KT 2 1/2SM -SN FEW021 BKN026 OVC045 M04/M09 A2979 RMK AO2 SNB15 P0000
Metar above SNB15 mean snow began at hour 15 or 1615z
Another important remark in metars is the SLP which stands for sea level pressure or MSL. Great when trying to find the center of systems. Just look for the SLP various metars. The first 1 or 2 digits are omitted. Here some examples
CYYG 162000Z 02009KT 5SM -RA BR OVC003 01/01 A2992 RMK SF8 SLP134
CYHZ 162019Z 35009KT 3/4SM R14/P6000FT/N R23/P6000FT/U -RA BR VV002 04/04 A2986 RMK FG8 SLP115
KBOS 161954Z 01004KT 1/4SM BR OVC003 04/03 A2989 RMK AO2 SFC VIS 1 1/2 SLP121 T00390028
KJFK 161951Z 02004KT 10SM BKN021 OVC031 05/M01 A2988 RMK AO2 SLP118 T00501011 $
CYYG SLP134 1013.4mb, CYHZ SLP115 1011.5MB, KBOS SLP121 1012.1MB, KJFK SLP118 1011.8MB
CYYG 150943Z 30009KT 1 3/4SM -RA BR SCT003 OVC010 05/05 A2946 RMK SF4SC4 PRESRR SLP979
In this metar we only omit 1 digit because. CYYG SLP979 is actually 997.9mb
I don’t know all the remarks for American metars but there must be a list somewhere. Canadian metars seem to usr remarks more often than US metars.
Forgot to mention that if a metar was issued in error or a mistake was made then a corrected report is sent using the abbreviation CCA or CCB.
Here is a corrected metar.
CYYG 161700Z CCA 04005KT 2SM -RA BR VCFG BKN003 OVC008 02/02 A2988 RMK FG3SF3SF2 VCFG VIS LWR NE-E PRESFR SLP120
On another note.. Canadian metars are always issued on the hour where as American metars are issued a few minutes prior.
Automatic stations use AUTO, or A02 etc