Humidity explainer: relative humidity, dew point, and what your hygrometer hides

Humidity is the environmental quantity laboratories measure most and understand least. Relative humidity is a ratio — water vapor present versus saturation at the CURRENT temperature — which makes it exquisitely temperature-sensitive: warm a room one degree and the RH falls several percent with no change in moisture.

That is why dew point is the quantity of record when moisture itself matters: it is temperature-independent, the temperature at which the present vapor would condense. A lab reporting 45 %RH ± 3 %RH at an uncontrolled temperature is often saying less than it thinks; the dew point pins the actual water content.

For calibration purposes, humidity instruments chain to temperature and pressure realizations — the CCT's domain — through humidity generators and chilled-mirror hygrometers. The library's SOP 49 covers calibration of environmental measurement standards, including the practical handling that keeps a hygrometer's certificate meaningful between calibrations: sensor conditioning, transport shock, and the drift that polymer sensors accumulate.