On the above plot, to simulate the image quality in an extreme condition with an outer scale equal to the telescope diameter, the tilt component of the 8m is removed from the DIMM FWHM. This approximate method was successfully tested on VLT-UT1 science images in conditions ranging from 0.3" to 1.5", ie. covering 95% of Paranal observable time.

20mn Long Exposure Seeing Paranal 1989-1995 (1998-2002)LaSilla 1989-1995 (1999-2002)
arcsec @ zenith, 0.5 microm. Infinite Outer Scale20m Finite Outer Scale Infinite Outer Scale20m Finite Outer Scale
25 percentile0.52 (0.64)0.40 (0.50)0.69 (0.70)0.54 (0.55)
50 percentile0.66 (0.82)0.52 (0.66)0.87 (0.89)0.70 (0.72)
Average 0.69 (0.91)0.54 (0.74)0.93 (0.97)0.76 (0.79)
75 percentile0.82 (1.08)0.66 (0.89)1.10 (1.15)0.91 (0.95)
Total number of samples12522 (31828)1252218559 (21156)18559
Fraction lower or equal to 0.5", %21 (9) 50 (27)5 (4)19 (18)

In the above table, the expected range of an 8m image quality is given for two values of the outer scale of the turbulence:
Infinite Outer Scale = DIMM seeing monitor original data
20m Finite Outer Scale = computed following a more accurate formula derived in Tokovinin, A., From differential image motion to seeing; PASP, 2002, V. 114, P. 1156
The actual improvement depends on the UT observing wavelength (much larger at large wavelengths) and zenith distance.

Seeing Variability at Paranal 1999-2002 (Ph. Gitton, ESO)

Statistics of the relative variability of the natural seeing at Paranal, made on 1mn samples for 20mn equivalent exposures.