Below is a graph of the seeing monitor of the Dutch Open Telescope. The seeing is measured using a scintillometer, which is a photodiode and an amplifier circuit that splits the signal into a DC and an AC component. The DC component (the green line in the graph) represents the mean intensity of the light received by the scintillometer, the AC component (the red line in the graph) is the RMS of the fluctuations of the intensity around the average. The larger the AC component, the more fluctuations there are, the worse the seeing is.
Although there is not a direct mapping between the scintillation signal and the image quality of the telescope, a value of -30 for the AC component represents good to very good image quality, a value of -25 medium quality, and -20 and higher bad to very bad image quality. One should not look so much at the average trend, but to the distribution of the signal. This is easy to see in the daily plots. Even though the average value might be close to -20, if there are lots of moments with seeing lower than -25, it is still possible to get good images.
A live stream of the data (in ASCII format, can be immediately used in gnuplot and many other plotting programs) is available from http://www.dot.iac.es/cgi-bin/seeing.
Plot of seeing since dawn today:
Plot of yesterday's seeing: