Decoding Meteorology Reports

Aviation weather reports are nothing if not concise.

The benefit of this is that a large amount of information can be passed quickly.   This morning I was having a flying lesson and I was listening to the radio conversations on the Cambridge Airport Approach frequency.  A pilot over-flying the airport requested a METAR weather report.   The controller was able to read the latest  METAR to the pilot in about 20 seconds.  The METAR contained the airfield identifier, date/time, surface wind speed/direction, visibility, a layered cloud report, temperature, relative humidity, barometric pressure and a significant weather report. That’s a lot of useful information passed in 20 seconds.

Here is an example of a METAR I just grabbed of the web.

SCCI 2416:10Z 22002KT 9999 SCT030 SCT200 06/M01 Q0993 NOSIG

It may look like gobbledygook-gook but that is jam packed with information.    If you can bear it, let’s go through it one bit at a time. Explaining the format to you helps me learn, so try and pretend to listen.

Airfield Identifier (SCCI)
This is a globally unique code, every airfield is allocated an  identification code by ICAO.  In Europe the first letter in the code is always a E and in Great Britian the second letter is always a G.  So for example Rothera is EGAR and Halley is EGAH.  In South America the first letter is S and in Chile the second letter is a C.  SCCI is the ICAO identifier code for Punta Arenas airport in Chile.  So we know that this report is for Punta airfield.

Date/Time (2416:10Z )
The first 2 characters are the day of the month and the next 4 are the time in 24H format.  The month and year are not included, they are assumed. The date/time is always in UTC which is know and Zulu time.  This is what the Z at the end means.  So we know from yesterdays blog post that local time in Punta is UTC-3 so this report was taken at 01:10pm local time of the 24th of September 2014(current month and year).

Wind (22002KT )
This is the wind direction in degrees magnetic (220) and the speed in knots (02).  So we know that the surface wind is a gentle 2 knot wind from the South-West.  Wind direction is always given in the direction that the wind is coming from.

Visibility (9999)
This is the horizontal visibility in metres. Anything over 10km is just reported as 9999 and typically read out as “all the nines” meaning more than 10km of horizontal visibility.  2000 for example would mean 2km.  If a visibility is reported in miles it will be postfixed with SM for statute mile ( Eg. 3SM)

Weather
Any noteworthy current weather is mentioned in between Visibility and Cloud.  This report does not include any weather codes.  These are reported as 2 character long code . Some of the common codes are:

HZ – Haze
BR – Mist
FG – Fog
TS – Thunderstorm
CB – Cumulonimbus
SN – Snow
GS – Small soft hail
GR – Hail
PE – Ice pellets
IC – Ice crystals (diamond dust)

The intensity of the weather will be reported as one of the following three:
– Light
Moderate (no symbol)
+ Heavy
If there is neither a + or – then the intensity is moderate.

Cloud (SCT030 SCT200)
There are two cloud reports in this METAR which means there are two cloud layers.  One low layer of scattered clouds at 3,000ft (SCT030)and a high layer of scatters cirrus at 20,000ft (SCT200).  Scattered means that 3-4 oktas (eighths) of the sky is covered by the cloud layer

Temperature and Humidity (06/M01)
The first part of this section (06) tells us that the current temperature is a chilly 6 degrees Celsius at Punta.  The second half (M01) is saying that the dew point is minus 1 degrees Celsius.  The dew point being the temperature at which the air will become saturated.  From this we  know that the humidity of the air is 7 degrees away from reaching saturation point. When air reaches saturation point it releases water vapour and that makes clouds.  Now very approximately the temperature drops 3 degrees for every 1000 feet of altitude gain.  You can climb a big hill and check this. So if it is 6 degrees on the surface we would reckon that at about 2,300ft higher it would be -1. So 01/M01 would suggest there might be a cloud layer at about 2-3,000 feet.  And hey looking back at the cloud report there is indeedy a SCT030 cloud report !

Pressure  (Q0993)
This is the QNH which is a pressure measurement taken at the surface then adjusted to  mean sea level to remove the airfield elevation. It is given in hector pascals and pilots can use this to calibrate their altimeters to ensure it show an accurate elevation in the current local atmospheric conditions.  1013hPa is considered a ‘normal’ QNH so 993hPa is quite low, it suggests that a low pressure system may well be in the area.  So in addition to needing this to set the calibration sub-scale on their altimeters a pilot would probably also notice that it was kinda low and maybe frown.

Expected Variations or trends(NOSIG)
This section of the report highlights any expected significant variations.  NOSIG means no significant trends expected in the next 2 hours.  Other trend codes include:
BECMG – Becoming, meaning a permanent change is expected
TEMPO-  Temporarily, meaning a temporary change lasting less than 1 hr is expected
NSW  – No Significant Weather
NSC – No Significant Change
If BECMG or TEMPO are included then these will be followed by a codified message describing the change.

CAVOK
Some times the visibility, cloud report and weather section are replaced with the word CAVOK, which means ceiling and visibility OK.   This substitution is only made if visibility is more than 10km, there is no cloud below 5,000ft and there is no significant weather.  So CAVOK is good news for pilots hoping for conditions suitable for visual flying .

 

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