Okay lets just jump right in here! Your asking yourself the question, what is a METAR? A METAR is an ground station observation taken at regular intervals at airports around the world. Not all airports issue METAR's, some smaller fields don't have a meteroligist on site or a automated weather station but out of all the airfields Eurostar fly too I have not come accross one yet that doesnt issue a METAR. A METAR is a useful tool during all stages of the flight, during pre-flight planing for checking arrival conditions. For instance if you are going on a short flight from Gatwick to Manchester, but Manchester is reporting a heavy thunderstorm, with raing and winds gusting to 50knots it may be advisable for your to consider if the flight is actually going to be able to land there! Also, during pre-flight it is useful for planning alternate airports. When flying ETOPS it is necessary for your diversions to meet specific levels in terms of weather, for instance if flying a single ILS approach then the cloudbase must be at least 600ft and visibility is 3km. Also enroute in normal conditions it can be useful to check arrival conditions for performance reasons at your landing airport.
METAR's are held in a coded form to save space. Lets look at an example
EHAM 220800z 16505KT 9999 FEW030 04/02 Q1024
without explaing further I shall now decode this METAR and I belive it will be pretty explanatory what each term means.
EHAM - Amsterdam Schipol
220800z - Day 22 at 0800z
16505KT - Winds are from 165° at 5 knots
9999 - Visibility is greater than 10km
FEW030 - Few clouds at 3000ft (Although note this time the type of cloud is not specified)
04/02 - Temperature is 04°C and dew point 02°
Q1024 - QNH, local pressure setting is 1024mB
In this very simple example is it quite easy to see where the terms come from. Although it may appear a bit daunting at first after a while coded METAR becomes easier to deal with than having it written out in long form. Of course this was a very simple example and they do get a bit more interesting than this!
EHAM 220955Z 21020KT 5000 -RADZ FEW006 BKN008 BKN015 03/03 Q1012 TEMPO 3500
EHAM - Amsterdam Schipol
220800z - Day 22 at 0955z
16505KT - Winds are from 210° at 20 knots
9999 - Visibility is 5km
-RADZ = Light Rain and Drizzle (- = Light, RA=Rain, Dz=Drizzle)
FEW006 - Few clouds at 600ft
BKN008 - Broken clouds at 800ft
BKN015 - Broken clouds at 1500ft
03/03 - Temperature is 03°C and dew point 03°
Q1012 - QNH, local pressure setting is 1012mB
TEMPO 3500 - Temporarily visbility reduced to 3500m
In this example you see examples of -RADZ which describe the type of precipitation in effect. There are a variety used including RA for rain, DR for drizzle, GR for hail, FG for fog, BR for mist, SN for snow.... you can see all examples of these by clicking here
Now here you try and have a go, decode the following METAR without looking at my breakdown and compare how you got
EHLW 221425Z 21019KT 1800 R24/1400N RADZ BR SCT004 BKN005 OVC008 04/04 Q1004
EHLW - Leeuwarden
221425Z - Day 22 at 1425z
21019KT - Winds from 210° at 19 knots
1800 - Visibility 1.8km
R24/1400N - Runway 24 visibility 1400m
RADZ - Rain Drizzle
BR - Mist
SCT004 - Scattered clouds at 400ft
BKN005 - Broken clouds at 500ft
OVC008 - Overcast at 800ft
04/04 - Temperature 04°C and dew point 04°C
Q1004 - QNH 1004mB
How did you do? If you got most of it your onto a good start! As you can see there are many variations that are added, bits that are changed. More information about European METAR's can be found here
The bigest difference can be found in a METAR from the USA... Look at this one below
KTLH 160752Z 17011G23KT 3SM +TSRA BR FEW017 BKN025CB 16/15 A2986 RMK A02 WSHFT 0653 PRESRR SLP111 P0046 T01560150
KTLH - Tallahassee Reg. FL
160752z - Day 16 at 0752z
17011G23KT - Winds from 170° at 11 knots gusting 23 knots
3SM - Visiblity is 3 stature miles
+TSRA - Heavy thunderstorm rain
FEW017 - Few clouds at 1700ft
BKN025CB - Broken clouds at 2500ft of type cumulonimbus type
16/15 - Temperature 16°C and dew point 15°C
A2986 - Altimiter 29.86 inches of mercury
RMK - Remark
AO2 - Automated weather station with precipitation discrimination
WSHFT 0653 PRESRR - Windshift at 0653z caused by pressure rising rapidly
SLP111 - Sea level pressure of 1011.1hPa
P0046 - 0.046inches of precipitation fell in the last hour
T01560150 - Temperature is 15.6°C and dew point is 15.0°C
As you can see the american METAR's contain much more information than the usual worldwide METARs. More information on american METAR's can be found here
Worldwide METAR data access can be found from http://weather.noaa.gov/weather/metar.shtml