  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 winter (November -
April) it is about twice higher than in summer (May - October). This
observational rate shows a large sinusoidal annual variation, and it seems to
have relation with the El Ni\~no and La Ni\~na phenomena; the La Ni\~na years
tend to have high observational rates, but the El Ni\~no 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.

Instrumentation and Methods for Astrophysics