I had heard of METAR several times before, but never really took the time to understand it until I began to code the Groundhog application. To be able to download and understand the weather data, my program had to understand METAR....which meant I had a little research to do. My understanding of METAR is a collection of data that I found on a few sites explaining METAR, including the National Weather Service.

  A METAR report is basically a weather observation report that is issued on an hourly basis. METAR is the standard code that is used to format the report. The reports are available from an anonymous FTP site, ftp://tgftp.nws.noaa.gov/data/observations/metar/stations. In this directory will see multiple files that follow this naming convention: XXXX.TXT. The 'XXXX' is what is known as the ICAO code. This is a 4 letter code that represents locations of weather observation stations. For instance, I use the location code KBMG, which represents Bloomington, IN. Below is a METAR report for KBMG. There are more possibilites than what is detailed below, but these are the basics and allow us to decode this sample report. Let's tear it apart and explain what each piece of data means.


Explanation:

This is the date and time at which the observation data was taken. In a way this is a bit redundant, as you will see a little later. I'm not sure why the time is repeated later.

This is the 4-letter ICAO code. In this example, KBMG is Bloomington, IN.

This is also the date and time at which the observation data was taken. This does vary from the observation time at the beginning of the file. The first two digits are the day number, the last four digits are the zulu (UTC) time. To convert to Eastern Standard time use: (UTC - 5 hours). To convert to Eastern Daylight Time use: (UTC - 4 hours).

This designates that the observation was taken automatically. If this designation does not appear, it means that someone was involved in the recording the data.

This data represents the speed and direction of the wind. The first three digits indicate the direction of the wind (90 is east, 180 is south, 270 is west). The next two digits represent windspeed, measured in knots. This particular variable can change. For example 18015G50KT would represent winds out of the south at 15 knots with gusts of 50knots. For variable winds 6 knots and below, the direction is replaced by VRB. For variable winds, winds that vary by more than 60 degrees, you may see a wind reading like this: 27010KT 215V315. This simple means that the winds are out of the west at 10 knots and that it varies in direction from 215 degrees to 315 degrees. If the winds are calm the reading would be 00000KT.

This represents visibilityin US, Statute Miles(SM) and Fractions. If visibiity is less than 1/4 statute miles, it will be reported as M1/4SM.

This represents the Sky Condition. In this example, the condition is clear. This tells us that there are no clouds below 12,000 feet. Here are some more possibilities:

Few = 1/8 to 1/4 cloud cover
SCT = 3/8 to 1/2 cloud cover
BKN = 5/8 to 7/8 cloud cover
OVC = 100% cloud cover

The following three digits represent cloud height in hundreds of feet. For example 010 would be 1,000 feet. If they are present, these numbers will be followed by a 'T' for towering cumulus or 'CB' for cumulonimbus.

These numbers tell us the temperature and dewpoint seperated by an '/'. The first two numbers are the temperature in celsius and the second two numbers are the dewpoint in celsius. If either of these readings fall below 0 degrees celsius, they will be proceeded by an 'M'. For example, M05/M08.

This is the barometric pressure. In this example it is 29.99 inches of mercury.

RMK(remarks) designates the end of the METAR observation and means that what follows are remarks concerning the observation.

A01 means that it is an automated station without a precipitation sensor. A02 means that it is an automated station with a precipitation sensor.

This designates the sea level pressure.

This is the hourly temperature and dewpoint in tenths of a degree celsius. The first '0' represents either a positive or negative temperature. Positive is '0', while negative is '1'. The next three numbers are the temperature. The fourth number represents either a positive or negative dewpoint. The next three numbers are the dewpoint. In this example the temerature is 21.1C and the dewpoint is 13.9C.