things get worse faster than you'd think at the OSF because of two reasons: one is the increased PWV, but the other is that the wet and dry opacity terms are larger at the OSF - I ran ATM for both the 5000m and the 2900m site, and here are the values of those opacity terms for CHanjnantor and the OSF: CH_345_DRY = 0.017047 CH_345_WET = 0.151363 CH_650_DRY = 0.056474 CH_650_WET = 1.041826 OSF_345_DRY = 0.038981 OSF_345_WET = 0.214849 OSF_650_DRY = 0.123080 OSF_650_WET = 1.449320 tau_CH_345 = OSF__345_DRY + PWV * OSF_345_WET Looking at this, it seems a bit suspicous that the Dry terms did more than double in coming down to the OSF. The wet terms increased by like 30-50%, that seems right. The other part of my assumption, that the PWV goes up by a factor of exp((5000-2900)/1800) = 3.2 -- this seems like it may be excessive. When you think about it, on a good day the water vapor that passes over the OSF isn't going to condense into clouds over the high site, but rather it seems that a lot of the PWV that passes over the OSF will also pass of the high site, so I would hedge a bit on this factor of 3.2 higher PWV. Chajnantor Tau 345GHz Tau 650GHz month 10% 25% 50% 10% 25% 50% 01 0.1448 0.2994 0.6122 0.9355 2.0000 4.1531 02 0.1905 0.3346 0.6755 1.2500 2.2419 4.5886 03 0.1237 0.2045 0.3732 0.7904 1.3468 2.5080 04 0.0920 0.1307 0.2467 0.5726 0.8388 1.6371 05 0.0850 0.1131 0.1799 0.5243 0.7178 1.1775 06 0.0710 0.0991 0.1659 0.4275 0.6210 1.0807 07 0.0745 0.0956 0.1377 0.4517 0.5968 0.8871 08 0.0710 0.0920 0.1588 0.4275 0.5726 1.0323 09 0.0780 0.1026 0.1553 0.4759 0.6452 1.0081 10 0.0850 0.1131 0.1694 0.5243 0.7178 1.1049 11 0.0710 0.0920 0.1377 0.4275 0.5726 0.8871 12 0.0780 0.1237 0.2151 0.4759 0.7904 1.4194 OSF -- assuming scale height of 1.8 km - the water vapour will be 3.2 times higher at the OSF than at the high site; also, using a 2900m site for the dry and wet opacity terms Tau 345GHz Tau 650GHz month 10% 25% 50% 10% 25% 50% 01 0.6191 1.3215 2.7424 4.0363 8.7750 18.3599 02 0.8266 1.4812 3.0298 5.4364 9.8519 20.2984 03 0.5233 0.8905 1.6568 3.3901 5.8672 11.0366 04 0.3796 0.5552 1.0821 2.4209 3.6055 7.1595 05 0.3477 0.4754 0.7787 2.2055 3.0671 5.1133 06 0.2838 0.4115 0.7149 1.7747 2.6363 4.6825 07 0.2998 0.3956 0.5872 1.8824 2.5286 3.8209 08 0.2838 0.3796 0.6829 1.7747 2.4209 4.4671 09 0.3158 0.4275 0.6670 1.9901 2.7440 4.3594 10 0.3477 0.4754 0.7308 2.2055 3.0671 4.7902 11 0.2838 0.3796 0.5872 1.7747 2.4209 3.8209 12 0.3158 0.5233 0.9384 1.9901 3.3901 6.1902 ============================================================== So, even with hedging a bit on the factor of 3.2 * PWV, it seems that we can do 345 GHz at the OSF to some extent most nights in the winter months, but we will almost never be able to do anything at 650 GHz at the OSF.