the High pressure is not exactly above Hong Kong, but here we enjoy long hours of clear sky and good seeing
stormsurfing on global jetstream wind and 250mb pressure
stormsurfing on Jetstream Asia
subtropical high is also a very important factor
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 no observation is possible with a seeing index of "5", if cloud cover is 100%. Conversely, it is possible that there is no cloud cover 0% (and all stars are "visible"), but observation is severely reduced by low seeing index ("1"), due to air turbulence.
There are 2 reasons for not including the cloud cover in the seeing index calculation:
1. Seeing is a state of the air - it is therefore independent of cloud cover.
2. Sometimes, clouds are sparse, and then observations can be made between clouds.
In this case, it would be very difficult to set a "limit" for cloud cover which influences the Seeing Index and satisfies all users.
留言
張貼留言