This seems good advise about seeing in general


jimcoble2000
 

The very best seeing often comes on still, muggy summer nights when the air is heavy with humidity and the sky looks unpromisingly milky with haze. Some astronomers claim that a blanket of industrial smog steadies the air as effectively as summer humidity — or rather that it results from the same tranquil air masses.

Time of night also plays a role, but again there are few universal rules. Right after sunset the seeing is apt to be excellent, so start your planetary observing as soon as you can find a planet in twilight. The seeing is apt to deteriorate before dusk fades out. Some observers find that their seeing improves after midnight; others say it goes to pieces. This depends largely on local topography; observers in valleys might get worse seeing as the night goes on and cold air flows down to pool in the valley. Just before sunrise may be another excellent time.

For observing the Sun (use an astronomer's solar filter!), the best time is early morning before the Sun heats the landscape. The very worst atmospheric seeing of the 24-hour daily cycle comes in the afternoon.

Geography is critical. Smooth, laminar airflow is the ideal sought by observatory-siting committees worldwide. The best sites on Earth are mountaintops facing into prevailing winds that have crossed thousands of miles of flat, cool ocean. You don't want to be downwind of a mountain; the airstream breaks up into turbulent swirls after crossing the peak. Nor do you want to be downwind of varied terrain that absorbs solar heat differently from one spot to the next. Flat, uniform plains or gently rolling hills extending far upwind can be almost as good as an ocean for providing laminar airflow. You may learn to predict which wind direction brings the best seeing to your observing site.


From S&T

I think the first paragraph is quite astute. Also as the humidity goes up the ability of the atmosphere to change temperature goes down as per my last post. See the "heat capacity of water". In fact when fog form the temperature of the atmosphere is "locked in". It can't change. You are at what is called an invariant point. Two states of matter are simultaneously present (gas and liquid) That is also why fog is great for planets if not your clothing and sanity. The air temp can't change until the sun starts to evaporate air moisture; then at lower water content the air temp can change again.