• atmospheric effects;
  • site testing


We present comprehensive seeing statistics for the San Pedro Mártir site derived from the Thirty Meter Telescope site-selection data. The observations were obtained between 2004 and 2008 with a Differential Image Motion Monitor (DIMM) and a Multi-Aperture Scintillation Sensor (MASS) combined instrument (MASS–DIMM). The parameters that are statistically analysed here are the whole atmosphere seeing (measured by the DIMM), the free atmosphere seeing (measured by the MASS) and the ground-layer (GL) seeing, which is the difference between the total seeing and the free atmosphere seeing. We have made a careful data coverage study, along with statistical distributions of simultaneous MASS–DIMM seeing measurements, in order to investigate the nightly, monthly, seasonal, annual and global behaviour, as well as possible hourly seeing trends. Although this campaign covers five years, the sampling is uneven, with 2006 and 2007 being the best sampled years in terms of seasonal coverage. The overall results yield a median seeing of 0.78 arcsec (DIMM), 0.37 arcsec (MASS) and 0.59 arcsec (GL). Therefore, the strongest contribution to the whole atmosphere seeing comes from a strong ground layer. We find that the best season is summer, while the worst season is winter, in accordance with previous studies. It is worth noting that the best yearly results are correlated with the best sampled years. The hourly analysis shows that there is no statistically significant tendency for seeing degradation towards dawn. The seeing values are slightly larger than those reported previously, which might be the result of climate changes.