the fwhm of the seeing disc (or just "seeing") is usually measured in arcseconds, abbreviated with the symbol (″). a 1.0″ seeing is a good one for average astronomical sites. the seeing of an urban environment is usually much worse. good seeing nights tend to be clear, cold nights without wind gusts. warm air rises (convection), degrading the seeing, as do wind and clouds. at the best high-altitude mountaintop observatories, the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4".