  From 1991 until 1997, the 3.8m UK Infrared Telescope (UKIRT) underwent a
programme of upgrades aimed at improving its intrinsic optical performance.
This resulted in images with a FWHM of 0."17 at 2.2 um in September 1998. To
understand and maintain the improvements to the delivered image quality since
the completion of the upgrades programme, we have regularly monitored the
overall atmospheric seeing, as measured by radial displacements of subaperture
images (i.e. seeing-generated focus fluctuations), and the delivered image
diameters. The latter have been measured and recorded automatically since the
beginning of 2001 whenever the facility imager UFTI (UKIRT Fast Track Imager)
has been in use.
  In this paper we report the results of these measurements. We investigate the
relation between the delivered image diameter and the RMS atmospheric seeing
(as measured by focus fluctuations, mentioned above). We find that the best
seeing occurs in the second half of the night, generally after 2am HST and that
the best seeing occurs in the summer between the months of July and September.
We also find that the relationship between Zrms and delivered image diameter is
uncertain. As a result Zrms frequently predicts a larger FWHM than that
measured in the images.
  Finally, we show that there is no correlation between near-infrared seeing
measured at UKIRT and sub-mm seeing measured at the Caltech Submillimetre
Observatory (CSO).
