the High pressure is not exactly above Hong Kong, but here we enjoy long hours of clear sky and good seeing 15 Jul 2021, 13:30 Christopher Go on Jetstream in youtube below 1:42:28 stormsurfing on global jetstream wind and 250mb pressure stormsurfing on Jetstream Asia subtropical high is also a very important factor hko meteoblue . explanation of seeing calculation : Seeing Index 1 and Seeing Index 2 are two different models to calculate the visibility, and are independent of the cloud cover. These values are computed based on the integration of turbulent layers in the atmosphere. They only represent the visibility through assumed clear portions of air, and differ in the way of how the atmospheric density gradients influence the visibility. Seeing 2 gives more weight to the effect of density fluctuations, and is more likely to indicate air "flickering" due to turbulence. Why seeing does NOT include the cloud cover. It is possible that ...