From Indian Skies #10 Where do you put a submillimeter telescope? The answer depends on one thing above all: how much water vapor sits above your telescope. Even a few millimeters can block the Universe at these wavelengths. 1/

Submillimeter telescopes study cold gas, star formation, black holes, and dusty distant galaxies. But atmospheric water vapor absorbs this light extremely efficiently. So the world’s best observatories are built at very high, very dry sites. 2/

The @ALMA Observatory📡 in Chile – currently the world's leading submm telescope – sits at ~5000 m elevation and routinely sees Precipitable Water Vapour (PWV) near or below 1 mm. Could Ladakh compete? 3/

Singh, Rao & Thakur analyzed 15 years of ERA5 atmospheric data across the Ladakh plateau, computing monthly precipitable water vapor (PWV) maps from 2010–2025. They then modeled the resulting submillimeter transmission. 4/

They also tackled a subtle problem. Global atmospheric models overestimate PWV at mountain observatories because coarse-resolution grids include moist low-altitude air that doesn’t actually lie above the telescope. So the best site might be missed due to an oversight! 5/

Using Hanle radiometer data, the authors corrected for this effect. What did they find? Current optical telescope sites Hanle and Merak achieve PWV ≤1 mm only ~5–8% of the time. But two eastern Ladakh regions perform much better. 6/

Site A: ~23% overall, ~70% in winter Site B: ~19% overall, ~50% in winter In the important 850 μm atmospheric window, Site A reaches winter transmission ~0.80 — compared to ~0.83 at APEX in Chile. 7/

At lower ALMA bands, Ladakh winter conditions become surprisingly competitive with conditions at ALMA — the world’s leading submillimeter observatory. 8/

What's next? This is NOT a telescope announcement. ERA5 has ~25 km resolution, while Himalayan terrain changes dramatically within each pixel. A nearby ridge could be significantly drier – or wetter – than the average value. 9/

The next step is clear: deploy radiometers at Site A and Site B for a full annual cycle and measure the real sky directly. And if ERA5 is systematically overestimating PWV in Ladakh, the actual sites may be even better than these already-impressive results suggest. 10/

@Raman Research Institute , Bengaluru / Arizona State University / Caltech–JPL Singh, Rao & Thakur, The Astronomical Journal, 171, 331 (2026) https://arxiv.org/abs/2604.13487 11/

‘From Indian Skies’ highlights recent astrophysics research from Indian institutions, written for everyone. I select papers in my personal capacity from my interests and desire for variety, nothing else. Paper authors are typically not involved in these posts. 12/