We have quantitative criteria for such things as the difference between VMC and IMC. The Met office (of any country) provides us with plan view charts to help us determine where is which.
One of those met offices (the UK one, in this matter) is producing an areal chart which delineates a certain parameter (volcanic ash density, in this case), but it is becoming extremely difficult to obtain a factual statement of what the criterion is which is used in drafting those areal limits.
Can anybody give me a straight answer?
What density is being used to draw those red lines?
Is it different to the criterion which was being used the day before yesterday?
If so, why? and by how much?