How to Decode PART of a METAR Weather Observation - v.971117
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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:
 
KPIT 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:  KPIT 211151Z
 
       KPIT     This is the International Civil Aviation Organization
                (ICAO) identifier for Pittsburgh International Airport.
                (See the ICAO Identifier Table for cross reference.)
 
       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 occassionally 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).
 
        -SN     Present weather is light snow. (See table 3 below.)
 
        FEW021  The lowest cloud layer is at 2,100 feet (021)
        BKN027  AGL (AGL means "Above Ground Level"), the next
        OVC035  is at 3,500' AGL (035), and so on.  (See table 1
                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 preceeded 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 preceeded 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 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 table 2 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).