A correct and thorough analysis of current and expected weather situations is of uttermost importance in aviation. To assist a pilot in performing a correct weather risk assessment prior to his/her flight, meteorological offices around the world will produce a wide range of meteorological products:
However, a good weather analysis prior to departure is only half the work!
Throughout a flight, a pilot should remain vigilant for rapidly deteriorating weather situations. Only recently (April 2018) the Federal Aviation Administration (FAA) indicated that a failure to recognise deteriorating weather, continues to be a frequent cause or contributing factor of accidents in General Aviation.
Monitoring the ATIS (Automatic Terminal Information Service), VOLMET (en-route ATIS) and requesting PIREPs (Pilot Reports), can and will assist a pilot in staying ahead of the weather whilst en-route!
Many of these weather products are published in an abbreviated aviation language.
At first glance, this might seem rather complex and not user-friendly. Furthermore, nowadays many online decoders are available which will rapidly translate a weather product into plain language. Tempting but....don’t use the easy way!
Firstly this meteorological “slang” and corresponding symbols are easy-to-use. Secondly, a thorough understanding of these items remains crucial to ensure a safe outcome of your flight.
In today’s article we will discuss some important abbreviations used in Meteorological Aerodrome Reports (METAR) and we’ll zoom in on the purpose of the color codes they might include.
A metar describes the actual meteorological situation at an aerodrome and its vicinity.
Published at half-hourly intervals, it will contain information concerning temperature, dew point, wind direction and speed, precipitation, cloud cover and heights, visibility and barometric pressure. Furthermore information on precipitation amounts, lightning and colour states might be included.
If between the 30 minute regular intervals a significant deterioration or improvement in airport conditions occurs, a Special Aerodrome Report (SPECI) will be published.
The example metar shown below, is from Antwerp Airport (EBAW). It was published on the 28th of April at 14:20 UTC. When discussed briefly, the following information can be derived from this report.
Remember that the reported windspeed is an average value, measuredduring the previous 10 minutes with reference to True North. If this averagevalue is exceeded by a minimum of 10 knots, a gust factor (“G”) will be included.
Only when the wind direction varies by 60 degrees or more in the previous 10 minutes, the “V” symbol and wind extremities will be included. A calm or variable wind is indicated by the abbreviation “VRB”.
A horizontal visibility of 3 kilometers is indicated by “3000” while a horizontal visibility of less than 50 meters is referred to as “0000”.
Common weather abbreviations are outlined in the table below.
Remember that the cloud coverage will always be expressed in “oktas".As illustrated in the picture below, a meteorologist will divide the sky in 8 parts or “oktas”. Labelling a cloud coverage as scattered, sky clear, overcasts, etc. will depend on the amount of sections covered with clouds.
Remember that, whenever the prefix M is included, this refers to a negative temperature. For example 00/M03 indicates a temperature of 0 °C and a dewpoint of minus 3°C.
Although a metar gives an indication about actual weather prevailing, it can include a TREND indicator. These abbreviations will provide insights into how the weather will change in the next 2 hours.
The abbreviations used in a weather report are not solely limited to those stated in the example metar above. Other commonly used abbreviations are discussed below.
“CAVOK” indicates “Clouds/Ceiling and Visibility are OK” and is reported when:
“NSW” indicates “No Significant Weather” and is used to indicate the end of significant weather.
“VV” refers to “Vertical Visibility” and is provided when the visibility is very bad and no cloud base can be determined. It measured from the ground upwards.(E.g. VV003 indicates a vertical visibility of 300 feet.)
“RE” is used to indicate (significant) “recent weather” observed during the last routine observation. (E.g. RETS refers to recent thunderstorms.)
“AUTO” indicates that the weather report has been produced using date from anautomated observing system without interference from a meteorologist.
Metars produced by military Met Offices often include color states, possibly in combination with trend indicators, to provide information on present and expected weather conditions.
These color codes (blue, white, green, yellow, amber and red) have a specific meteorological significance and will allow for a rapid assessment of actual and expected weather conditions.
The applicable color code is determined by comparing the lowest cloud base AAL observed with the worst surface visibility measured. The worst condition of either cloud base or visibility determines the applicable color code.
For example; a cloud base of 5000 feet and a visibility of 4 kilometers, will still imply a green color code. This despited the fact that the observed cloud base would imply a blue color code!
It is important to note that in the UK, Belgium, The Netherlands and France, the lowest cloud base AAL, is defined as 3 oktas or more. In the USA and other parts of Europe, the lowest cloud base implies 5 oktas or more.
For easy reference, one can assume that color codes ‘white’ and ‘blue’ imply VFR conditions. All other color codes will imply IFR conditions.
In some cases a trend indicator can be included. For example, a color code ‘BLUE WHITE’ signifies that the color state will turn white in the next 2 hours.
Furthermore, if the word ‘BLACK’ precedes the color state (e.g. BLACKWHT) , the runway in use is unusable due to other conditions than clouds or low visibility. (e.g. ice, snow, obstructions, etc.)
Please share your stories and experiences about this topic in the comment section below.