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station plot description Weather stations all over the world report weather conditions every hour using a data format referred to as METAR (this is a French acronym with a loose English translation to "routine aviation weather observation"). These data are collected centrally by the U.S. National Weather Service (and other country's equivalents) and distributed. The Surface data page provides a FORM interface so that you can retrieve these data in their raw format or you may choose to translate them to a more human-friendly format. See the accompanying stations.txt file for a list of worldwide airports that report METARs. For example, knowing that Denver, Colorado's airport identifier is "KDEN", you can simply retrieve the latest 24 hours worth of METARs by entering that string (without the double-quotes) into the text FORM field. You may enter as many stations as you like and simply separate them using a space and/or comma but you must use the full 4-letter ICAO abbreviation. Alternatively, you may mix with state abbreviations of the form "@CO" (upper and/or lower case are both valid). So, for example, you can enter the following: KDEN KAKO KSNY KBFF @WY   And, if you own a 3Com Palm Pilot, you can also get this same data wireless. See this link for more details.
    Meteorologists in the U.S. are accustomed to viewing the surface station data in a format like that shown here. Unfortunately Fahrenheit is still commonly used in the U.S. for temperature and dewpoint information. The field found to the upper right of each station location is mean sea-level pressure (MSLP) as of April 1, 2002. Prior to this date, the value shown was altimeter setting. With either MSLP or altimeter, the leading digit(s) is dropped (9 or 10 in the case of MSLP and 2 or 3 in the case of altimeter). In other words, a MSLP value of 983.6 mb is plotted as "836" while 1021.3 is plotted as "213". The graphic here is self-descriptive but for aviation purposes the cloud coverage amount is colorized red for Instrument Flight Rule conditions (IFR), magenta for low-IFR (LIFR), green for Visual Flight Rules (VFR) and blue for Marginal VFR. (see table below)
    By clicking on the wind barb on this graphic, you can view another page containing a wind barb description. Furthermore, the weather symbols used by meteorologists are quite cryptic (and in bad need of modernizing by the World Meteorlogical Organization) and a full table of these graphics is obtained by clicking the Present Weather label on the graphic.
  • Flight category definitions:
    Category Ceiling   Visibility
    Low Instrument Flight Rules
    LIFR* (magenta circle)
    below 500 feet AGL and/or less than 1 mile
    Instrument Flight Rules
    IFR (red circle)
    500 to below 1,000 feet AGL and/or 1 mile to less than 3 miles
    Marginal Visual Flight Rules
    MVFR (blue circle)
    1,000 to 3,000 feet AGL and/or 3 to 5 miles
    Visual Flight Rules
    VFR+ (green circle)
    greater than 3,000 feet AGL and greater than 5 miles
    *By definition, IFR is ceiling less than 1,000 feet AGL and/or visibility less than 3 miles while LIFR is a sub-category of IFR.
    +By definition, VFR is ceiling greater than or equal to 1,000 feet AGL and visibility greater than or equal to 3 miles while MVFR is a sub-category of VFR.

  • Cloud coverage symbols:
    Sky symbols
    Automated stations report "CLR" when clouds may exist above 12,000 feet so a square is used to represent this uncertainty whereas an unfilled circle is used for "SKC" which a human reports the sky is completely clear overhead. The abbreviation "OVX" is unofficial but we use it here to indicate the sky is obscured which is the case when a METAR reports vertical visibility and no cloud information.
Present Weather symbols
wx icons Plots of station data may include one of the following symbols to represent the present weather. METARs may include more than one type of weather condition but only one icon ever gets plotted. Two additional documents show the original descriptions abridged from the World Meteorological Organization (WMO) and which symbol you will find for which METAR present weather text string. These are not necessarily official but they represent my best matching of weather conditions to the original WMO table of icons.
Original WMO table and abridged descriptions of symbols:
wxSymbols_anno1.pdf (287 KB).
Text string for present weather found in METARs mapped to icons and plotting priority order:
wxSymbols_anno2.pdf (313 KB).
Wind Barb
Wind barb graphic Wind barbs are simply a conventient way to represent both wind speed and direction in a compact graphical form. Vectors also work to some degree but it is more difficult to discern the magnitude when viewing vectors. For this reason, meteorologists prefer the use of wind barbs. The graphic here clearly shows how to read a wind barb. Meteorologists are also accustomed to nautical miles per hour (knots) for the magnitude of the wind. Convert to statute miles per hour (mph) by adding 15% to the value in knots. Example: 60 knots = 60 + 9 mph. [Just remember to figure it the same way you would figure a 15% tip at a restaurant by taking 10% and then halve that value   ;-)   ]
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