Fibre Optic Sensors (FOS) based on Fibre Bragg Gratings (FBG) increasingly find application for long distance, distributed monitoring of temperature and strain. The sensors are immune to magnetic fields and electromagnetic noise; are tolerant to high radiation level; can be read-out over km distances; and several tens of point-measurements can be precisely interrogated in multiplexing over a single fibre. Following the successful installation of a network of ~100 temperature and strain sensors in CMS, a new class of FBG-based Relative Humidity sensors have been developed. Coupled to a FBG thermometer, this has been developed into the first FBG thermo-hygrometer ever produced. At the end of a research spanned over few years, the first network of 72 FBG thermo-hygrometers has been recently installed in CMS, marking the start of the field application of this new product for distributed monitoring of absolute humidity levels in the most environmentally demanding regions of HEP experiments.