A first frequency of electromagnetic radiation and a second frequency of electromagnetic radiation are received at a detector. The first and second frequencies of electromagnetic radiation are transmitted through a medium within a frequency range of approximately 0.1 TeraHertz to approximately 10 TeraHertz. Signals are generated that are proportional to the transmittance of the frequencies of electromagnetic radiation through the medium. A ratio of the of the signals is formed, and one or more of a relative humidity, an absolute humidity, and a water vapor concentration of the medium are calculated as a function of a temperature of the medium, the ratio, and a set of functional parameters associated with the temperature of the medium.