Visible volcanic ash: Setting the limit or not?

After the prolonged disruption to aviation caused by volcanic ash injected in atmosphere via volcano Eyjafjallajökull (Iceland) in April and May 2010 aviation society in Europe established the system of three volcanic ash concentration zones (low, medium and high) based on correlation between concentration level and safety and allowed flight operations in areas of low concentration. Nevertheless, airspace unusable for operators and manufacturers is still demarcated by the presence of “visible ash” but the clear definition of that term is unfortunately not yet defined. In order to contribute to determining and using of the term “visible ash” the video that shows visibility level in the vicinity of volcano Eyjafjallajökull after the eruption in June, 2010 is used for the discussion. Several different snapshots are presented and information generated out of them used as the input data for a mathematical calculation of the visibility situation of the volcanic ash concentration boundaries upon which internationally recognized volcanic ash concentration zones are defined. In the end, a preliminary visibility threshold for flight operations in areas contaminated with volcanic ash is proposed and discussed.