an extensive area of ash generally between 15,000ft and 20,000ft
is mentioned in the latest report but as far as I can see the word 'distinct' (as in 'distinct layer') does not occur. This phrase DID get used in a previous flight report from 5th May. Who can say exactly how a VA cloud will move, EXCEPT based on precisely the atmospheric modelling already being done. OK - the vertical component looks like it
might be made more specific with more work - but I don't know what's going on in that part of the forest. Other people around here might....?
The extent to which VA forms 'distinct' layers only a few thousand feet thick, or more diffuse layers covering 'generally' FL150 to FL200 is a bit academic anyway. For practical purposes, any VA between ground level and FL200 in (say) a 50 mile radius around an airport should be suffiicient to close it. ATC cannot realisitically factor-in the presence of VA in a particular set of levels and small area(s ) when directing traffic. How could that
possibly work in practice. The resolution of the cloud model predictions looks to me pretty much as small as you're likely to get, combined with safe margins of error, in flying and control, AND coping with unexpected manoeuvres caused by an aircraft suddenly appearing in the wrong bit of sky!