Hi Martin.
I think that the answer to your basic question is probably no, sky temperature is not a direct measure of clarity, so will not necessarily indicate how good the imaging conditions are.
a few notes that might possibly be useful:
- the sensors respond to a wider spectrum than just the water vapour window, so local temperature radiation from air that is close to the sensor is also important. Thus, the most useful measure is not absolute sky temperature, but the difference between ambient and the sky.
- the relative sky temperature only tells how much water is up there (vapour or clouds), not how clear the sky is in the visible band. You can have a sky with lots of visible scattering (light dust, aerosols) that would be useless for imaging, but it will still look cold in a thermal sensor (indicating clear sky). Alternatively you could have a clear humid atmosphere that would be OK for imaging, but would appear to be relatively warm to a thermal sensor.
- cloud temperature can vary a lot since it depends on local temp at the cloud (ie altitude) and high clouds could conceivably appear cold enough to look like clear sky. However, clouds also reflect ground radiation and that apparently makes them appear warm enough to be sensed as clouds.
- sensor field of view and pointing angle can affect the results.
- there is presumably some relationship between humidity (sky temperature) and turbulence (seeing) - not sure and interested if anyone has any info.
useful reference maybe:
http://onlinelibrary.wiley.com/doi/1...2JD017881/full
regards Ray