A versatile implementation of the psychrometer technique as a learning opportunity in atmospheric physics courses

Even today the psychrometer technique, if properly implemented, is used as a calibration standard for humidity measurements. In order to simplify the cumbersome use of the classical instrument, we recently proposed an original configuration characterized by the unattended operation and real-time readout of temperature sensors. More recently, we have upgraded that system by applying the online data acquisition controlled by a LabVIEW code which also displays the final observable of relative humidity. The program implements the psychrometer algorithm, usually available only in the form of tables, and also the data recording on disk. We describe here how to properly build the instrument and how to guarantee its intrinsic accuracy (typical uncertainty within a few per cent) as well as all the details of the formulae used. The psychrometer, proposed as a project work to university students following new courses on meteorological instrumentation, was found to be a powerful source of learning opportunities. Although at first, the working principle of the instrument looks easy and offers an intuitive interpretation of the concept of humidity, later students become aware of how difficult requirements have to be satisfied in order to realize a reference standard. As a final verification, they are asked to guarantee that this psychrometer implementation strictly conforms to the official recommendations of the World Meteorological Organization (WMO). We present here a few examples of their activities in planning autonomously a series of checks and measurements.