ABSTRACT
The present chapter discusses features of water and water vapors, the main gas laws of physics, which can help to understand how a humidity level depends on the environment, and units used for humidity measurement. Water is a special substance. Water covers 71% of the Earth’s surface. It is vital for all known forms of life. Life, climate, and the weather all exist as they do because gaseous, liquid, and solid forms of water can coexist on the planet. There are many different ways to express humidity. On the basis of measurement techniques, the most commonly used units for humidity measurement are relative humidity (RH), dew/frost point (D/F PT), and parts per million (ppm). It should be noted that the humidity can be fully described with a single parameter (any of them) if the gas pressure and temperature are known, but many national humidity laboratories have chosen the dew point temperature as the primary humidity parameter for practical reasons: the realization of a dew point temperature scale provides a simple source of traceability in a wide range of humidity at a good uncertainty level.
