There's a depressing familiarity between the media's coverage of the volcanic ash cloud over Europe, and their coverage of the radioactive contamination released by the Chernobyl explosion in 1986. In both cases, the contaminant is treated as a Boolean variable, (something which is simply true or false), rather than something which is present at different levels of concentration. In both cases, the implicit media assumption is that if the contaminant is detectable at any level, then it must be hazardous.
Only some levels of radioactive contamination are hazardous, and the same principle applies to the concentration levels of volcanic ash. Whilst the levels of volcanic ash over Iceland will obviously be high, and whilst such levels of concentration have been proven to be hazardous to aircraft in the past, elsewhere the ash will be dispersed to lower levels of concentration by random diffusion and high-altitude winds. Yet look at any map of the volcanic ash distribution, and it is treated as a homogeneous blanket.
Perhaps the media, however, are simply following here the direction provided by the air traffic control and meteorological authorities, who also seem to be unforthcoming about the specific concentration levels involved. On Friday, I posed the following question to the Press Office of the UK's air traffic control service, NATS:
i) What concentration level of volcanic ash in the atmosphere is considered to be hazardous to aircraft? Presumably there is a specific figure, perhaps expressed in terms of grammes per metre cubed, such that if the concentration level is greater than that, then it is considered hazardous for aircraft to fly under these conditions.
(ii) What scientific method was used to establish this maximum safe concentration level?
(iii) What scientific method is used to assess the concentration levels of volcanic ash currently over Europe?
Suffice to say, I'm still awaiting their response...