You know the air is clear....

... when you look out the window and see a mountain you've never seen before.

Looking north-east from ICC, I could see a large mountain looming behind the Lantau Link suspension bridge (top left).   I had to look-up a map to identify it as Castle Peak (583 m) or Pui To Shan (杯渡山).

According to Wiki, "Pui To Shan was associated with a Buddhist monk Pui To Sim Shi (杯渡禪師).  In Cantonese, 'Pui' means cup, 'To' means water-crossing, 'Sim Shi' means Monk of Zen. Legend has it that the monk had travelled over water in a cup and finally reached the Castle Peak. He established a monastery on the hill."  (wiki link).    A more recent version of the legend has the monk simply walking over the smog.

Anyway the air today was unusually clear, with an official visibility of 30km, which seems about right as the Castle Peak is about 25km from ICC (that explains why we normally can't see it).

Even more amazing, the air quality at the nearest monitoring station at Mon Kok was actually classified 'good'.