Hi Ian and Dean,
Seeing is related to turbulence in the atmosphere, caused by thermal variations at different heights. The best that can be (cheaply) done with the GFS model data is to work with the wind speeds at 10m and at the Tropopause (Jetstream) levels. The lower layers (Boundary and Surface) of the atmosphere have a greater effect on Seeing, and the algorithm in SkippySky reflects this. The effect of the Jetstream has been downgraded quite a bit since SkippySky went live in Dec 2008.
Transparency is a measure of the amount of water vapour in the atmosphere. Water vapour molecules block out starlight, and effectively make things dimmer. So yes, you can have good Seeing in terms of little wind/turbulence, but the air can be chockers with water vapour at the same time.
Regards,
Andrew Cool
www.skippysky.com.au