EAS 380 September 4, 2008

Properly Decoding METAR Observations

Material Covered:

• Introduction to Forecast Game Displays and Standings
• METAR Decoding Tricks of the trade (See METAR Decoding for examples)
  1. Use the WMO portion of the guide (second half)
  2. Make sure to use proper units and covert to SI units when needed.
  3. Hourly observation is typically made 5-15 minutes before the hour. (will have the most data)
  4. Observation system
     1. A01 in remarks is Automated Weather Observation System. Has no current weather discriminator.
     2. A02 in remarks is ASOS System. Has a current weather discriminator.
     3. AUTO indicates that the machine is doing the observation without any human augmentation.
  5. Sky cover
     1. Heights are cloud base.
     2. Instrumentation limit: automated system only detects clouds up to 12,000 feet.
     3. Ceiling definition: lowest broken, overcast, or obscured layer.
     4. Obscured report, i.e. VV002, means cloud base is not visible. The height listed gives the vertical visibility in hundreds of feet.
  6. Obstructions to visibility
     1. PL now used instead of PE for sleet.
     2. BR from an ASOS is translated as light fog.
     3. All "column 3" weather codes have an intensity (- light; + heavy; no sign: moderate). The others do not!
     4. AWOS (A01) without human help has no weather discriminator.
  7. Temperature and Dew Point Temperature
     1. Reported in °C.
     2. "1" in hundreds place indicates negative value.
     3. Look at T-group when available.
     4. Express tenths digit when available, even if ".0" (significant digits)
  8. Altimeter Setting
     1. A method of reducing pressure to mean sea level using the Standard Atmosphere.
     2. All US airports include it (needed by pilots).
     3. Can be used for surface pressure analysis (and will be used in this course).
     4. Correct term is "altimeter setting" or "altimeter pressure."
  9. Selected remarks
     1. Three-hour pressure tendency ("5" group)
        1. Pressure tendency symbols are translated in Table 10 (inside back page) of Surface Station Model handout.
           1. Intended to be mnemonic.
           • Example: Interpret "3" tendency as falling, then rising, not rising then rising more quickly
           2. Includes the sign.
              1. 0-3: pressure higher than 3 hrs ago
              2. 4: steady pressure
              3. 5-8: pressure lower than 3 hrs ago
        2. State them correctly:
           1. Example: 53098 means Pressure tendency has fallen, then risen, with a net rise of 9.8 mb in the past 3 hrs.
           • Frequent bad translation: Pressure tendency has fallen, then risen 9.8 mb in the past 3 hrs.
     2. High and low temperature groups
        1. 24-hrs: A 9 digit group led by a "4" indicator (same convention as T-group)
        2. 6-hrs: 5-digit groups led by a "1" (high temperature) or "2" (low temperature). Reported at 0000, 0600, 1200, 1800Z
     3. Melted/liquid precipitation accumulation
        1. All report precipitation in hundreths of inch (no decimal point shown)
        2. Types:
           1. P-group: total in current hr (can recycle at different times for different stations; look at several during a rain event to make sure.)
           2. 6-group: total in either last 3 hrs (at 0300, 0900, 1500, 2100Z) or last 6 hrs (at 0000, 0600, 1200, 1800Z)
           3. 7-group: 24 hr total precipitation (usually reported at 1200Z)
     4. Precipitation began/ended
        1. If hour not given, assumed to be the hour of the observation
        2. Does not have intensity information

See METAR Decoding for examples

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Last updated: Thursday, September 4, 2008 1:22 PM

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