Astronomical seeing
The atmosphere is not a fixed optical medium. Blobs of air of different temperatures and densities cause the light from distant objects to micro-refract. This causes small deviations in the light path and, at relatively high frequencies, causes objects to appear distorted and fuzzy. The degree of distortion is quantified under the collective term ‘seeing’.
Seeing is complicated and varies greatly with location. In essence its effects occur in three layers of the atmosphere. At high altitude, a fastmoving corridor of air known as the jet stream can cause high-frequency jitters that seriously degrade the view. The effect is often likened to trying to read newspaper print placed at the bottom of a fast-moving stream of water!
At the mid-level, seeing effects come from the movement of air above and around large-scale topographic features. These could be hills or mountains, valleys or depressions, or large expanses of water. The latter can produce what’s called 'laminar flow', which settles the mid-layer effects quite dramatically. Being downwind of a hill or ridge, you will experience turbulence. Similarly, heat from a town or city will create air instability. If you are downwind of a heat source like this, again you will experience turbulence. At the bottom layer, the effects are caused by local features such as immediately adjacent houses, trees and fences. Large areas of tarmac surface will warm up during the day and release heat during the night. This may cause significant seeing issues too, depending on wind direction, so it’s best to avoid these and set up in grassier areas if possible. During the winter, home heating combined with poor insulation may create disturbed seeing due to hot domestic air mixing with cold air.