WATS (WAter vapour and temperature in the Troposphere and Stratosphere) was an ESA Earth Explorer candidate concept for globally measuring humidity through the troposphere and temperature through the troposphere and stratosphere. Its radio-occultation constellation would establish highly accurate, vertically resolved climatologies, support climate-variability and climate-change research, validate and improve atmospheric models, and demonstrate an active atmospheric-sounding method. ESA documents WATS as a research concept rather than an implemented programme.