  The Observatorio del Roque de los Muchachos (ORM), in the Canary Islands
(Spain), was one of the candidates to host the future European Extremely Large
Telescope (E-ELT) and is the site of the Gran Telescopio Canarias (GTC), the
largest optical infrared facility to date. Sky transparency is a key parameter
as it defines the quality of the photometry to be acquired in the astronomical
observations. We present a study of the atmosphere extinction at the ORM,
carried out after analysis of a database spanning more than 20 yr, to our
knowledge, the longest and most complete and homogeneous in situ database
available for any observatory. It is based on photometric measurements in the V
band and r' band (transformed to the V -band extinction coefficient kV) using
the Carlsberg Meridian Telescope (CMT). Clear seasonal variations that repeat
yearly are observed. The median value of kV is 0.13 mag/airmass; the mean value
has a maximum in the summer months (June- September), corresponding to the
season with maximum frequency of nights affected by dust or cirrus (~29% in
summer, but only ~13% during the rest of the year). Two volcanic eruptions took
place during the database baseline, which has enabled the study of the impact
of volcanoes on the global atmosphere extinction. For the 5 yr of available
information, we have estimated the average monthly weather downtime from the
CMT data log, obtaining a result (20.7%) in reasonable agreement with earlier
studies. The main conclusion of our study is that there is no significant
evidence from the CMT data for any secular changes in kV over the 20 yr
database baseline.
