NB all information is here is from public domain sources.
Well, there’s no real short answer.
But here’s a summary how “how it works”. Everything is taken from the ICAO Manual on Ash, which, if you want the bible on ash and aeroplanes, is what you should be reading. ICAO is the International Civil Aviation Organization, the UN body which sets standards for international air travel.
The world is covered by nine Volcanic Ash Advisory Centres, which retain the capabilities to detect ash via satellite and forecast its spread using “dispersion” computer models. They provide guidance to “meteorological watch offices”, which in turn produce “SIGMETs”, coded bulletins which are transmitted to pilots to warn them of dangerous weather or, as in this case, ash. Sigmets are normally valid for six hours at a time, but volcanic ash sigmets also contain an 18 hour “outlook” for the use of long-distance flights and flight planning. As a result, the Volcanic Ash Advisory messages extend to 18 hours ahead, to inform the outlook part of the sigmets.
Therefore, new advisory messages are produced every six hours during a major volcanic eruption. That’s why you are hearing in the media, every six hours, that the airspace closures are lasting another six hours. At the bottom of the messages you can view using the link above, you can read remarks about volcano (to convert FLs, flight levels, into feet, times by one hundred), and when the next update is due.
NATS, the National Air Traffic Service, controls UK airspace. It updates shortly after the Met Office, using the forecast data.
So, when will the ash disperse? When either the upper-level winds change and blow the ash away, or the volcano stops erupting (but even then it will take some time for the last ash to reach us, and then for what’s already up there to disperse). From purely eye-balling ECMWF upper-air forecasts, if anything the upper wind looks like it’s only go to blow straight from Iceland to the UK for at least the first half of the week, but towards next weekend will move away.
As far as the eruption itself is concerned, well, you can see on the volcano webcam, now skies in the area have cleared, that there’s still plenty coming out of the volcano. How long that lasts for is impossible to forecast. The Icelandic Met Service is providing plenty of information — click on the “eruption” link at the top of their home page.
What’s happening over the UK? The Met Office Icelandic volcano eruption page is being regularly updated, and the forecaster’s blog is very interesting reading, reporting that ash has been observed over the UK at 6,000 ft and lower.
The Natural Environment Research Council has also been probing the cloud with its research aircraft. Unfortunately that aircraft can only fly to 15,000 ft — the much larger and more capable FAAM aircraft cannot be used as it has been stripped of instruments ahead of a repaint.
There has also been confirmed deposits of ash. Sheffield University has confirmed that ash fell in Sheffield.
Why can’t I see anything? (Again, from the ICAO Manual on Ash) Volcanic ash is made mainly of tiny particles of silica, too small to be seen by the naked eye when suspended in the air. By the same token, they are too small to be detected by weather radar, whether ground based or on aircraft. Often the only way pilots know they have flown into an ash cloud is from the acrid smell, the abrasion of the windshield (making it look like it’s frosted), and in the worse case, by the loss of thrust as the ash clogs the engines!
What damage can this ash do? Ash is much harder and more abrasive than sand and “normal” dust. It can severely damage jet engines resulting on repairs that can cost millions. Indeed, some Finnish jet fighters have been damaged, and Easyjet is worried about engine damage is flight is allowed again too early.
How much is there really up there? This is partly answered above by the fact that ash has been detected in the air and on the ground in the UK, and the Finns have provided stark evidence that it is dangerous. However the ICAO manual does explain that forecasting actual concentrations is not possible—only where ash from a volcano will have spread too. Flight within the area where ash is present is dangerous, or to quote the manual, “AVOID AVOID AVOID”. As data from the research flights and from the Icelandic Met Service is sent to the Met Office, they will be able to refine the forecasts as much as possible.
And finally, who produces the most CO2? The planes, by a long way…