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:
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 ADDS uses it here to indicate the sky is obscured which is the case when a METAR reports vertical visibility and no cloud information.
The section of the Surface data page with this title provides an interface to overlay several individual graphics found on this web site for the purpose of severe weather forecasting. To use this section, you must have Java turned on and you should be able to get a page showing the latest satellite image for the region of Tornado Alley where you click and an overlay of the latest surface graphic. Then, you also have the choice to overlay several gridded numerical model fields (derived from surface analyses produced by the Rapid Update Cycle model). These choices include: moisture convergence and lifted index, convective available potential energy (CAPE) and convective inhibition (CIN), and vorticity generation potential. When interpreted properly, these fields can be extermely helpful in determining locations most likely to be affected by severe weather. I use them as tools to forecasting possible tornado-producing storms. Much-deserved thanks go to Paddy McCarthy for his work to produce the Java code found here. Note: this Java Applet does not work with Netscape on the Mac because the 4.x versions do not support Java 1.1. However, Internet Explorer for Mac does work.