Assmann Psychrometer determine the air temperature and humidity of the ambient air. They consist of two thermometers, each inside a double tube for minimizing radiative heating both by the direct sun and longwave radiation exchange between thermometer and surrounding tube. In addition the instrument is ventilated during measurement by a manually winded fan. One of the thermometers is covered by a cotton gaze that has to be wettened before the measurement. Depending on the ambient humidity, water is evaporating from the wet gaze and cooling down the wet thermometer that reaches the so called wet-bulb temperature after a few minutes. From the wet bulb temperature, the air temperature measured simultaneously by the dry thermometer and the air pressure, the humidity of the air can be calculated.