Abstract
Observational rates under terahertz (THz) opacities less than 3.0 and 2.0 at the Atacama Large Millimeter/submillimeter Array (ALMA) site have been calculated using the 225 GHz tipping radiometer monitoring data and the opacity correlation between 225 GHz and THz opacities. The observational rate with THz opacity condition less than 3.0 is 12.4% in a year, and in austral summer (November-April) it is about twice higher than in austral winter (May-October). This observational rate shows a large sinusoidal annual variation, and it seems to have relation with the El Niño and La Niña phenomena; the La Niña years tend to have high observational rates, but the El Niño years show low rates. On the other hand, the observational rate with the THz opacity condition less than 2.0 is only 1.9%, and no obvious annual and seasonal variations are observed. This indicates that THz observations under low opacity condition of less than 2.0 at the ALMA site are very difficult to be performed.
