The Indian Astronomical Observatory (IAO) located at Hanle near Leh in Ladakh is fast becoming one of the most promising observatory sites globally due to its advantages of more clear nights, minimal light pollution, background aerosol concentration and extremely dry atmospheric condition uninterrupted by monsoon.
Researchers from India and their collaborators carried out a detailed study of the night-time cloud cover fraction over eight high altitude observatories, including three in India, before concluding that Hanle is way ahead of others in popularity amongst the next generation of astronomical observatories.
The research, which has been published in the Monthly Notices for Royal Astronomical Society (MNRAS), was led by Shantikumar Singh Ningombam of Bangalore's Indian Institute of Astrophysics (IIA), scientists from Aryabhatta Research Institute of Observational Sciences (ARIES) in Nainital, autonomous institutes of government's Department of Science & Technology (DST) and collaborators from Bangalore's St. Joseph's College and the National Institute of Meteorological Sciences, South Korea, University of Colorado and Chemical Sciences Laboratory, NOAA, USA.
Astronomers are constantly searching for ideal locations around the world to build their next big telescope based on local meteorological data collected over many years. Such studies are crucial in planning for future observatories and the prediction of how they will vary with time.
The researchers used reanalysis data, combined from assimilation and observation extending over 41 years, along with 21 years of data from satellites.
The study classified the quality of observable nights for different astronomical usages like photometry and spectroscopy on a daily basis. They analysed datasets for the Indian Astronomical Observatory (IAO) in Hanle and Merak (Ladakh), Devasthal (Nainital) in India, Ali Observatory in the Tibet Autonomous Region in China, South African Large Telescope in South Africa, University of Tokyo Atacama Observatory and Paranal in Chile and the National Astronomical Observatory in Mexico.
The team found that the Hanle site which is as dry as Atacama Desert in Chile, and much drier than Devasthal, has around 270 clear nights in a year and is also one of the emerging sites for infrared and sub-mm optical astronomy. This is because water vapour absorbs electromagnetic signals and reduces their strength.
They found that Paranal, located in a high-altitude desert in Chile, is the best site in terms of clear skies with around 87% of clear nights in a year.
IAO-Hanle and Tibet's Ali observatory, located around 80 km from each other, are similar to each other in terms of clear night skies.
They found that Devasthal has a slightly larger number of clear nights compared to the other sites in the sub-continent but are affected by monsoons for about three months in a year.
However, night observations at IAO-Hanle from 2m-Himalayan Chandra Telescope (HCT) are possible throughout the year without any interruption due to monsoons.
The cloud cover fraction for Hanle, Merak, and Devasthal in India and Ali in Tibet were 66-75%, 51-68%, 61-78%, and 61-75%, respectively at various time resolutions using satellite-era and reanalysis data.
Studying the variation of atmospheric parameters from 1980 to 2020 for all sites, the researchers found decreasing trends for the cloud cover fraction over the central region of Africa, the Eurasian continent, and the American continents.
There was however an increasing trend witnessed in the maritime region as well as land regions in the Sahara Desert, the Middle East, Indian sub-continent, the Tibetan Plateau and some of the Southeast Asian islands. This was possibly caused by global warming and associated water vapour changes in land-oceanic regions.
Such detailed studies of long-term cloud fraction and other various meteorological parameters help IIA plan for several mega-science projects such as National Large Solar Telescope (NLST) of 2-meter aperture and National Large Optical Telescope (NLOT) of 8-10 meter aperture at Merak and Hanle, respectively in the high-altitude Ladakh region.
"Such detailed analysis of data from multiple sites over many years and the prediction of their variation with time are crucial in order to plan for future observatories," says Ningombam.
After examining several years of data of various astro-climatological parameters, IIA had installed the 2-meter aperture Himalayan Chandra Telescope (HCT) at Indian Astronomical Observatory (IAO), Hanle. Thereafter, due to the uniqueness of this site, several astronomical telescopes operating at optical and infrared wavebands have been installed at Hanle by several Institutes of the country.
Also Read: Ladakh’s Tso Kar Wetland Complex now a Wetland of International Importance
