Pace:
The MK1 eyeball is the best way of detecting particles either water or dust. Clear air is clear air. I find it hard to understand claims of invisible.
Pollution in dust form is visible to pilots either as a haze or thin cloud and usually with a slight colouration.
I take your point on looking vertically but horizontally NO unless of course your in cloud or flying at night.
of course you are right: a well trained or experienced eye will see the ash in the clear air. It has been cleary demonstrated by german and british research flights where lidar
measurements were taken.
The question is how to standardise or validate the eye sightings against any definite level of aceptable ash contents in the air. The right answer was given in the documentation from the engine manufacturesre: avoid ANY area are where there is detectable ash. And since clouds do happen even in England
the NATS/CAA approach was justified.