We present the 3.5-yr monitoring results of 225 GHz opacity at the summit of the Greenland ice sheet ( Greenland Summit Camp ) at an altitude of 3200 m using a tipping radiometer .
We chose this site as our submillimeter telescope ( Greenland Telescope ; GLT ) site , because conditions are expected to have low submillimeter opacity and because its location offers favorable baselines to existing submillimeter telescopes for global-scale Very Long Baseline Interferometry ( VLBI ) .
The site shows a clear seasonal variation with the average opacity lower by a factor of two during winter .
The 25 % , 50 % , and 75 % quartiles of the 225 GHz opacity during the winter months of November through April are 0.046 , 0.060 , and 0.080 , respectively .
For the winter quartiles of 25 % and 50 % , the Greenland site is about 10 \% - 30 \% worse than the Atacama Large Millimeter/submillimeter Array ( ALMA ) or the South Pole sites .
Estimated atmospheric transmission spectra in winter season are similar to the ALMA site at lower frequencies ( < 450 GHz ) , which are transparent enough to perform astronomical observations almost all of the winter time with opacities < 0.5 , but 10 \% - 25 \% higher opacities at higher frequencies ( > 450 GHz ) than those at the ALMA site .
This is due to the lower altitude of the Greenland site and the resulting higher line wing opacity from pressure-broadened saturated water lines in addition to higher dry air continuum absorption at higher frequencies .
Nevertheless , half of the winter time at the Greenland Summit Camp can be used for astronomical observations at frequencies between 450 GHz and 1000 GHz with opacities < 1.2 , and 10 % of the time show > 10 \% transmittance in the THz ( 1035 GHz , 1350 GHz , and 1500 GHz ) windows .
Summer season is good for observations at frequencies lower than 380 GHz .
One major advantage of the Greenland Summit Camp site in winter is that there is no diurnal variation due to the polar night condition , and therefore the durations of low-opacity conditions are significantly longer than at the ALMA site .
Opacities lower than 0.05 or 0.04 can continue for more than 100 hours .
Such long stable opacity conditions do not occur as often even at the South Pole ; it happens only for the opacity lower than 0.05 .
Since the opacity variation is directly related to the sky temperature ( background ) variation , the Greenland Summit Camp is suitable for astronomical observations that need unusually stable sky background .