Ash clouds threaten air traffic

[Coireall]

Rubik, you have hit the nail on the head.
My reference:
Met Office: Volcanic Ash Advisory Centres
The latest bullitin from the Volcanic Ask Advisory Centre in London shows the observed cloud from Surface to FL200 to stretch from the eastern Atlantic to the Urals and from the tip of Scandanavia to the south of central Europe. If Civil aviation is not to stay grounded for the years that this volcano may continue to spew out its ash and particles, a "Command Judgement" will have to be made soon to get airborne again.
The best estimate of the plume height is FL280. Clearly the plume and its hazards must be avoided.
However, the density of the ash now spread over such a vast area is diminishing all the time. Moreover, there is now plenty of evidence that the ash is reaching the surface. However, the remarks section of the warning states: "ASH CONCENTRATIONS WITHIN INDICATED AREAS ARE UNKNOWN. NO SIGNIFICANT RISK ABOVE FL350".
We cannot stay grounded forever while ash is airborne. We need a much better risk analysis tool and than simply an ash cloud diagram containing a million square mile area which has to be avoided by all aircraft.