Why are millimetre-wave radio telescopes built on high mountains?

Millimetre-wave radio telescopes are constructed on high mountains or at other high-altitude locations primarily to ensure clear skies and dry conditions.1

At these high elevations, the telescopes are positioned above a significant portion of the Earth’s atmosphere, which is essential for minimizing the absorption and distortion of incoming signals. Specifically, observations at these short wavelengths are significantly affected by water vapour and oxygen present in the atmosphere. By operating in high, dry environments, astronomers can reduce these atmospheric effects and minimize phase distortions that would otherwise interfere with the quality and precision of the data collected.123