Where is Ridge A?

First identified in 2009 as the site with the lowest precipitable water vapour (PWV) and best terahertz transmission on Earth, ``Ridge A'' is located approximately 150 km south of Dome A, Antarctica. We use three years of data from the Microwave Humidity Sensor (MHS) on the NOAA-18 satellite and recent ground-based measurements from Ridge A to probe the PWV variations and stability over the high Antarctic plateau.