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How to read a Volcanic Ash Advisory 4 December 2009

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The Volcanic Ash Advisories issued by the network of Volcanic Ash Advisory Centres are a vital element in aviation safety, and provide valuable information on volcanic activity around the globe. These reports are technical aviation documents and can appear difficult to understand. The aim of this article is to make them a little clearer. VAAC terminology is actually simple and straightforward, and the format of Volcanic Ash Advisories is standardized, so once you can read one you can read them all.

Below is the text of a recent Volcanic Ash Advisory issued by Washington VAAC for Soufrière Hills volcano in Montserrat (original document here), with explanatory annotations in square brackets below each section.

FVXX20 KNES 031726
VA ADVISORY
DTG: 20091203/1726Z

[The first line need not concern us, and the second simply identifies this as a Volcanic Ash (VA) Advisory. DTG is the date and time of the advisory, given in the format YYYYMMDD followed by the time in the 24-hour format HHMM. The 'Z' at the end is the abbreviation for Zulu or Universal Time (Greenwich Mean Time). Thus this advisory was issued on 3 December 2009 at 17:26 UT/GMT]

VAAC: WASHINGTON
VOLCANO: SOUFRIERE HILLS 1600-05
PSN: N1642 W06210
AREA: W_INDIES
SUMMIT ELEV: 3002 FT (915 M)
ADVISORY NR: 2009/271

[This section identifies: (1) the VAAC issuing the advisory, which in this case is Washington; (2) the volcano to which the advisory pertains, Soufrière Hills in Montserrat, with its Volcano Number, which is 1600-05; (3) the position of the volcano, given in decimal latitude and longitude; (4) the area where the volcano is located; (5) the summit elevation in feet and metres; and (6) the number of this advisory in the series issued this year for this volcano.]

INFO SOURCE: GOES-12. GFS WINDS. VOLCANO WEB CAMERA.
ERUPTION DETAILS: ONGOING EMISSIONS

[INFO SOURCE summarizes the information used to compile this advisory, in this case the GOES-12 satellite, Global Forecast Service wind data, and webcam imagery. ERUPTION DETAILS gives the nature of the activity being reported, e.g. ongoing or intermittent emissions, gas/ash venting.]

OBS VA DTG: 03/1645Z
OBS VA CLD: SFC/FL120 40NM WID LINE BTN N1643 W06213 – N1605 W06509 – N1608 W06736. MOV W 21KT
FCST VA CLD +6HR: 03/2300Z SFC/FL120 40NM WID LINE BTN N1643 W06214 – N1613 W06735.
FCST VA CLD +12HR: 04/0500Z SFC/FL120 40NM WID LINE BTN N1643 W06214 – N1613 W06737.
FCST VA CLD +18HR: 04/1100Z SFC/FL120 40NM WID LINE BTN N1643 W06214 – N1610 W06736.

[This section summarizes the location, extent and potential future movement of the volcano's ash emissions, according to the best information the VAAC has. OBS is the abbreviation for 'observed', FCST for 'forecast'; VA stands for 'volcanic ash' and CLD, as you might expect, for 'cloud'. The date and time the emission was observed is given (here 03/1645Z, which is 3 December at 16:45 UT/GMT), followed by the altitude and position of the ash cloud at that time. This is followed by forecasted altitudes and positions for 6, 12 and 18 hours ahead. Position is given by decimal latitude and longitude and altitude is expressed by 'flight level', FL, followed by a number giving the altitude in units of 100 feet (e.g. FL120 = 12000 feet) Where possible the upper and lower altitudes at which an ash cloud may be encountered are given, so that an ash cloud situated between 10000 and 12000 feet would be reported as FL100/FL120. In this case the base of the cloud is reported as being at the surface ('SFC'), and the top of the cloud as reaching no higher than 12000 feet: SFC/FL120. When, as is very often the case, the altitude of the base of an ash cloud is unknown it has become a convention to report it as SFC, although you will sometimes seen the term UNKN ('unknown') instead.]

RMK: A 30-40 NMI WIDE PLUME OF LIGHT ASH IS MOVING AT A MEASURED 21 KTS AND EXTENDS ABOUT 250 NMI TO THE WEST OF THE VOLCANO. NO HOTSPOT HAS BEEN SEEN AND THE HEIGHT APPEARS TO BE AT, OR LOWER THAN FL120 BASED ON SOUNDINGS AND MODEL WINDS. NO SIGNIFICANT CHANGES ARE ANTICIPATED. …BALDWIN

[A section labelled RMK or 'remarks' will sometimes be included, containing further comments. Some VAACs are chattier than others.]

NXT ADVISORY: WILL BE ISSUED BY 20091203/2330Z

[Finally, the expected date and time of the next advisory for this particular volcanic event is given. If the activity has ended and no further advisories appear to be necessary, this line will read NO FURTHER ADVISORIES.]

The Volcanic Ash Advisories issued by the nine VAACs are most conveniently accessed via this NOAA page. Their individual websites are as follows:

The Volcanism Blog

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