Correct Response: A. The traditional sling psychrometer consists of two thermometers attached to a handle in a way that allows the thermometer to be spun rapidly. The two thermometers are identical except that one of the thermometers is covered in a wick that is moistened (the wet-bulb thermometer). As the device is spun, the water in the wick evaporates and evaporative cooling reduces the temperature of the wet-bulb thermometer. The difference in the temperature recorded by the two thermometers is used to calculate the relative humidity. Apparent temperature (B) is perceived temperature and its calculation takes into account relative humidity and wind speed, but it is not related to the working principle of a psychrometer. Air mass density is not an underlying principle upon which a psychrometer works (C) and air-pressure is not involved in psychrometer calculations or the underlying principle of the psychrometer's operation (D).
