How to Decode PART of a METAR Weather Observation - v.971117 ============================================================ When looking at weather observations, it is important to realize that a single weather observation represents the conditions existing during one particular time at one specific location. For a routine weather observation the time period is from 15 minutes before the hour to the transmission time (usually about 5-10 minutes before the hour). Most observations are routine. Any other transmission time (see below) indicates a "special" observation. Reported conditions may or may not be regional in scope. For instance, rainfall reported at KLAX is not necessarily the same as the amount received in the Los Angeles region. It is just what was received in a six inch wide circle (a rain gauge) at Los Angeles International Airport (KLAX). A METAR observation (an "obs") can be as simple as: KSBP 231447Z 00000KT 25SM SKC 05/04 A3027= Or it can be more complicated: KBOS 211151Z 25009KT 1 1/2SM -SN FEW021 BKN027 OVC035 M12/M15 A3014 RMK A02 SLP236 8/546 P0000 60000 T11171150 11106 21131 51016= It consists primarily of four parts, the last of which (Remarks) doesn't always appear. Part 1 Identification: KBOS 211151Z KBOS This is the International Civil Aviation Organization (ICAO) identifier for Boston's Edward Lawrence Logan International Airport. 21 Observation is for the 21st day of the month. 1151Z The time the observation ended (and was transmitted) in this case 1151 UTC (this is the 12Z or 12 GMT obs). Note: If "AUTO" appears just after the time group, the observation is from an automated station. Part 2 Observations: 25009KT 1 1/2SM -SN FEW021 BKN027 OVC035 25009KT Wind is coming from 250 degrees from true north (250 degrees clockwise from true north) at 009 nautical miles per hour (knots). (1 kt = 1.15 mph) If this wind had been gusting to 20 knots, it would have been coded 25009G20KT. 1 1/2SM The visibility is reduced to 1 1/2 (or 1.5) statute miles (SM) because of light snow (-SN). (Any visibility less than 7 statute miles must indicate what is causing the reduction.) The visibility will occasionally be followed by something like R28L/2000FT. This is a "runway visual range" (runway visibility) observation. The RVR reading above says "runway 28L visibility is 2000 feet. M2000 says "less than 2000" (minus) and "P2000" says "greater than 2000" (plus). "RVR" values are reported in three stages; [1] runway visual range at touchdown point, [2] runway visual range at midpoint, [3] runway visual range at roll out -SN Present weather is light snow. (See Note 3/Weather below.) -FEW021 The lowest cloud layer is at 2,100 feet (021) AGL -BKN027 The next cloud layer is broken at 2,700 feet (027) AGL -OVC035 The final cloud layer is overcast at 3,500 feet (035) AGL (See Note 1/Sky Cover below for further explanation.) Part 3 Gauge Readings: M12/M15 A3014 M12 The air temperature is -12 C (+10 F). M15 The dew point temperature is -15 C (+5 F). (The dew point is a measure of atmospheric moisture and the relative humidity is computed using the air temperature and dew point.) A3014 The altimeter setting (a measure of barometric pressure) is 30.14 inches of mercury. Part 4 Remarks and Coded Data: This can be lots of things (snow depth, amount of precipitation, pressure tendency, etc.) with most in code. It is preceded by "RMK". A01/A02 This is from an automated station. A01 does not have the capability to detect precipitation. A02 has a "precipitation discriminator". P0000 The total precipitation received during the last hour. P0000 indicates 0/100's of an inch and P0017 would indicate 17/100's of an inch. T11171150 The hourly air and dewpoint temperatures to the nearest 1/10 C degree. 1117 is -11.7 C (coded M12 above) and 1150 is -15.0 (coded M15 above). A positive number is preceded by a 0 instead of a 1 (0117 would be +11.7 C). SLP236 The sea level pressure (SLP) is 1023.6 mb (millibars or 102.36 pascals). This is another measure of atmospheric pressure. A low number (like 236 [23.6]) must be added to 1000 millibars (indicating 1023.6 mb) while a high number must be added to 900 mb (so 978 [97.8] indicates 997.8 mb). 11106 The 6 hour maximum temperature (the highest air temperature recorded during the previous six hours) is -10.6 C. The first 1 is the group identifier, the second is the sign (0 for + or 1 for -), and the final three digits are the temperature (106 is 10.6). 21131 The 6 hour minimum temperature (-13.1 C) coded as above. Here, the 2 is the group identifier. 4/001 The total snow depth on the ground in inches. Usually found in the 06 and 18Z observations. 411061131 A nine digit group beginning with a 4 as the group identifier would contain the 24 hour maximum and minimum temperatures, in that order, coded as in the "T" group above. Usually found in the 08Z observation. 51016 The "5" group is the 3 hour pressure tendency and amount of change. The second digit is the tendency (coded, where 0-3 are going up, 4 is steady, and 5-8 are going down) and the last three digits are the change (016 is 1.6 mb). 60000 3 and 6 hour precipitation amounts encoded as above. 60217 would indicate 2.17 inches. The 3 hourly precip is reported in the 70025 The 24 hour total precipitation (this will be liquid equivalent for frozen precip) in 1/100s of an inch. 70025 would be 0.25 inch. This is usually found in the 12Z observation. 8/546 This identifies the low, middle, and high cloud types using WMO code. (See Note 2/Sky Cover below.) 933125 Liquid water equivalent of the snow on the ground in 1/10s of an inch. 933125 says the SWE is 12.5 inches. Usually in the 18Z obs. 98096 Duration of sunshine in minutes. 98096 means there were 96 minutes of sunshine during the day. Usually in the 08Z observation. Plus a lot of other information that can be encoded or given in plain language (ex: volcanic eruptions, wind shifts, precipitation beginning or ending, lightning type and direction, etc.). Note: (1) Sky Cover: SKC/CLR Clear (no clouds) FEW Few clouds (1 to 2 eighths [1/8 to 2/8] of the sky is covered with clouds at this level). SCT Scattered (3/8 to 4/8 cloud cover). BKN Broken (5/8 to 7/8 cloud cover). OVC Overcast (8/8 or all the sky is covered). Note: (2) Cloud Types: Code Low Clouds Middle Clouds High Clouds ---- --------------- --------------- --------------- 0 None None None 1 Cu (fair wx) As (thin) Ci (filaments) 2 Cu (towering) As (thick) Ci (dense) 3 Cb (no anvil) Ac (thin) Ci (often w/Cb) 4 Sc (from Cu) Ac (patchy) Ci (thickening) 5 Sc (not Cu) Ac (thickening) Ci/Cs (low in sky) 6 St or Fs (fair) Ac (from Cu) Ci/Cs (hi in sky) 7 Fc/Fs (bad wx) Ac (w/Ac,As,Ns) Cs (entire sky) 8 Cu and Sc Ac (w/turrets) Cs (partial) 9 Cb (T-storm) Ac (chaotic) Cc or Cc/Ci/Cs Ac-Altocumulus, As-Altostratus, Cb-Cumulonimbus, Cc-Cirrocumulus, Ci-Cirrus, Cs-Cirrostratus, Cu-Cumulus, Fc-Fractocumulus, Fs-Fractostratus, Ns-Nimbostratus, Sc-Stratocumulus, St-Stratus (wx = weather) Note: (3) Weather: Intensity: - Light intensity (moderate is blank) + Heavy intensity VC In the vicinity Descriptor: MI Shallow / PR Partial / BC Patches / DR Low drifting / BL Blowing / SH Showers / TS Thunderstorm FZ Freezing Precipitation: RA Rain / DZ Drizzle / SN Snow / SG Snow Grains / IC Ice Crystals / PE Ice Pellets / GR Hail /GS Small Hail / UP Unknown Precipitation Obscuration: FG Fog / VA Volcanic Ash / BR Mist / HZ Haze / DU Widespread Dust / FU Smoke / SA Sand / PY Spray Other: SQ Squall / PO Dust or Sand Whirls / DS Duststorm / SS Sandstorm / FC Funnel Cloud (Tornado or Waterspout) This METAR decode was prepared by the Climate Research Division of Scripps Institution of Oceanography. Anyone having corrections or comments please contact Larry Riddle (lriddle@ucsd.edu).